fix: require google-auth>=1.25.0 (#190)

diff --git a/google/api_core/grpc_helpers.py b/google/api_core/grpc_helpers.py
index 5937f18..e8f9ee8 100644
--- a/google/api_core/grpc_helpers.py
+++ b/google/api_core/grpc_helpers.py
@@ -17,7 +17,6 @@
 import collections
 
 import grpc
-from packaging import version
 import pkg_resources
 import six
 
@@ -44,11 +43,6 @@
     except pkg_resources.DistributionNotFound:  # pragma: NO COVER
         _GOOGLE_AUTH_VERSION = None
 
-if _GOOGLE_AUTH_VERSION is not None and version.parse(_GOOGLE_AUTH_VERSION) >= version.parse("1.25.0"):
-    _GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST = True
-else:
-    _GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST = False
-
 # The list of gRPC Callable interfaces that return iterators.
 _STREAM_WRAP_CLASSES = (grpc.UnaryStreamMultiCallable, grpc.StreamStreamMultiCallable)
 
@@ -232,38 +226,19 @@
         )
 
     if credentials_file:
-        # TODO: remove this if/else once google-auth >= 1.25.0 is required
-        if _GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-            credentials, _ = google.auth.load_credentials_from_file(
-                credentials_file,
-                scopes=scopes,
-                default_scopes=default_scopes
-            )
-        else:
-            credentials, _ = google.auth.load_credentials_from_file(
-                credentials_file,
-                scopes=scopes or default_scopes,
-            )
+        credentials, _ = google.auth.load_credentials_from_file(
+            credentials_file,
+            scopes=scopes,
+            default_scopes=default_scopes
+        )
     elif credentials:
-        # TODO: remove this if/else once google-auth >= 1.25.0 is required
-        if _GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-            credentials = google.auth.credentials.with_scopes_if_required(
-                credentials,
-                scopes=scopes,
-                default_scopes=default_scopes
-            )
-        else:
-            credentials = google.auth.credentials.with_scopes_if_required(
-                credentials,
-                scopes=scopes or default_scopes,
-            )
-
+        credentials = google.auth.credentials.with_scopes_if_required(
+            credentials,
+            scopes=scopes,
+            default_scopes=default_scopes
+        )
     else:
-        # TODO: remove this if/else once google-auth >= 1.25.0 is required
-        if _GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-            credentials, _ = google.auth.default(scopes=scopes, default_scopes=default_scopes)
-        else:
-            credentials, _ = google.auth.default(scopes=scopes or default_scopes)
+        credentials, _ = google.auth.default(scopes=scopes, default_scopes=default_scopes)
 
     if quota_project_id and isinstance(credentials, google.auth.credentials.CredentialsWithQuotaProject):
         credentials = credentials.with_quota_project(quota_project_id)
@@ -271,16 +246,9 @@
     request = google.auth.transport.requests.Request()
 
     # Create the metadata plugin for inserting the authorization header.
-
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if _GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        metadata_plugin = google.auth.transport.grpc.AuthMetadataPlugin(
-            credentials, request, default_host=default_host,
-        )
-    else:
-        metadata_plugin = google.auth.transport.grpc.AuthMetadataPlugin(
-            credentials, request
-        )
+    metadata_plugin = google.auth.transport.grpc.AuthMetadataPlugin(
+        credentials, request, default_host=default_host,
+    )
 
     # Create a set of grpc.CallCredentials using the metadata plugin.
     google_auth_credentials = grpc.metadata_call_credentials(metadata_plugin)
diff --git a/setup.py b/setup.py
index 5de5aaf..26fab7e 100644
--- a/setup.py
+++ b/setup.py
@@ -31,7 +31,7 @@
 dependencies = [
     "googleapis-common-protos >= 1.6.0, < 2.0dev",
     "protobuf >= 3.12.0",
-    "google-auth >= 1.21.1, < 2.0dev",
+    "google-auth >= 1.25.0, < 2.0dev",
     "requests >= 2.18.0, < 3.0.0dev",
     "setuptools >= 40.3.0",
     "packaging >= 14.3",
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 1fcd193..ff5b4a7 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -7,7 +7,7 @@
 # Then this file should have foo==1.14.0
 googleapis-common-protos==1.6.0
 protobuf==3.12.0
-google-auth==1.21.1
+google-auth==1.25.0
 requests==2.18.0
 setuptools==40.3.0
 packaging==14.3
diff --git a/tests/asyncio/test_grpc_helpers_async.py b/tests/asyncio/test_grpc_helpers_async.py
index 3461cbe..868018c 100644
--- a/tests/asyncio/test_grpc_helpers_async.py
+++ b/tests/asyncio/test_grpc_helpers_async.py
@@ -18,7 +18,6 @@
 import pytest
 
 from google.api_core import exceptions
-from google.api_core import grpc_helpers
 from google.api_core import grpc_helpers_async
 import google.auth.credentials
 
@@ -276,11 +275,7 @@
 
     assert channel is grpc_secure_channel.return_value
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        default.assert_called_once_with(scopes=None, default_scopes=None)
-    else:
-        default.assert_called_once_with(scopes=None)
+    default.assert_called_once_with(scopes=None, default_scopes=None)
     grpc_secure_channel.assert_called_once_with(target, composite_creds)
 
 
@@ -306,14 +301,8 @@
 
     assert channel is grpc_secure_channel.return_value
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        default.assert_called_once_with(scopes=None, default_scopes=None)
-        auth_metadata_plugin.assert_called_once_with(mock.sentinel.credentials, mock.sentinel.Request, default_host=default_host)
-    else:
-        default.assert_called_once_with(scopes=None)
-        auth_metadata_plugin.assert_called_once_with(mock.sentinel.credentials, mock.sentinel.Request)
-
+    default.assert_called_once_with(scopes=None, default_scopes=None)
+    auth_metadata_plugin.assert_called_once_with(mock.sentinel.credentials, mock.sentinel.Request, default_host=default_host)
     grpc_secure_channel.assert_called_once_with(target, composite_creds)
 
 
@@ -332,12 +321,7 @@
 
     grpc_helpers_async.create_channel(target, ssl_credentials=ssl_creds)
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        default.assert_called_once_with(scopes=None, default_scopes=None)
-    else:
-        default.assert_called_once_with(scopes=None)
-
+    default.assert_called_once_with(scopes=None, default_scopes=None)
     composite_creds_call.assert_called_once_with(ssl_creds, mock.ANY)
     composite_creds = composite_creds_call.return_value
     grpc_secure_channel.assert_called_once_with(target, composite_creds)
@@ -360,12 +344,7 @@
 
     assert channel is grpc_secure_channel.return_value
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        default.assert_called_once_with(scopes=["one", "two"], default_scopes=None)
-    else:
-        default.assert_called_once_with(scopes=["one", "two"])
-
+    default.assert_called_once_with(scopes=["one", "two"], default_scopes=None)
     grpc_secure_channel.assert_called_once_with(target, composite_creds)
 
 
@@ -386,12 +365,7 @@
 
     assert channel is grpc_secure_channel.return_value
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        default.assert_called_once_with(scopes=None, default_scopes=["three", "four"])
-    else:
-        default.assert_called_once_with(scopes=["three", "four"])
-
+    default.assert_called_once_with(scopes=None, default_scopes=["three", "four"])
     grpc_secure_channel.assert_called_once_with(target, composite_creds)
 
 
@@ -417,12 +391,7 @@
 
     channel = grpc_helpers_async.create_channel(target, credentials=mock.sentinel.credentials)
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        auth_creds.assert_called_once_with(mock.sentinel.credentials, scopes=None, default_scopes=None)
-    else:
-        auth_creds.assert_called_once_with(mock.sentinel.credentials, scopes=None)
-
+    auth_creds.assert_called_once_with(mock.sentinel.credentials, scopes=None, default_scopes=None)
     assert channel is grpc_secure_channel.return_value
     grpc_secure_channel.assert_called_once_with(target, composite_creds)
 
@@ -440,12 +409,8 @@
     channel = grpc_helpers_async.create_channel(
         target, credentials=credentials, scopes=scopes
     )
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        credentials.with_scopes.assert_called_once_with(scopes, default_scopes=None)
-    else:
-        credentials.with_scopes.assert_called_once_with(scopes)
 
+    credentials.with_scopes.assert_called_once_with(scopes, default_scopes=None)
     assert channel is grpc_secure_channel.return_value
     grpc_secure_channel.assert_called_once_with(target, composite_creds)
 
@@ -464,12 +429,7 @@
         target, credentials=credentials, default_scopes=default_scopes
     )
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        credentials.with_scopes.assert_called_once_with(scopes=None, default_scopes=default_scopes)
-    else:
-        credentials.with_scopes.assert_called_once_with(scopes=default_scopes)
-
+    credentials.with_scopes.assert_called_once_with(scopes=None, default_scopes=default_scopes)
     assert channel is grpc_secure_channel.return_value
     grpc_secure_channel.assert_called_once_with(target, composite_creds)
 
@@ -508,12 +468,7 @@
         target, credentials_file=credentials_file
     )
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=None, default_scopes=None)
-    else:
-        google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=None)
-
+    google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=None, default_scopes=None)
     assert channel is grpc_secure_channel.return_value
     grpc_secure_channel.assert_called_once_with(target, composite_creds)
 
@@ -536,12 +491,7 @@
         target, credentials_file=credentials_file, scopes=scopes
     )
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=scopes, default_scopes=None)
-    else:
-        google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=scopes)
-
+    google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=scopes, default_scopes=None)
     assert channel is grpc_secure_channel.return_value
     grpc_secure_channel.assert_called_once_with(target, composite_creds)
 
@@ -564,12 +514,7 @@
         target, credentials_file=credentials_file, default_scopes=default_scopes
     )
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=None, default_scopes=default_scopes)
-    else:
-        google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=default_scopes)
-
+    google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=None, default_scopes=default_scopes)
     assert channel is grpc_secure_channel.return_value
     grpc_secure_channel.assert_called_once_with(target, composite_creds)
 
@@ -585,12 +530,7 @@
 
     grpc_helpers_async.create_channel(target, credentials=credentials, scopes=scopes)
     grpc_secure_channel.assert_called()
-
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        credentials.with_scopes.assert_called_once_with(scopes, default_scopes=None)
-    else:
-        credentials.with_scopes.assert_called_once_with(scopes)
+    credentials.with_scopes.assert_called_once_with(scopes, default_scopes=None)
 
 
 @pytest.mark.asyncio
diff --git a/tests/unit/test_grpc_helpers.py b/tests/unit/test_grpc_helpers.py
index 4e0ab80..8c6202b 100644
--- a/tests/unit/test_grpc_helpers.py
+++ b/tests/unit/test_grpc_helpers.py
@@ -234,11 +234,7 @@
 
     assert channel is grpc_secure_channel.return_value
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        default.assert_called_once_with(scopes=None, default_scopes=None)
-    else:
-        default.assert_called_once_with(scopes=None)
+    default.assert_called_once_with(scopes=None, default_scopes=None)
 
     if grpc_helpers.HAS_GRPC_GCP:
         grpc_secure_channel.assert_called_once_with(target, composite_creds, None)
@@ -268,15 +264,8 @@
 
     assert channel is grpc_secure_channel.return_value
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    print(grpc_helpers._GOOGLE_AUTH_VERSION)
-    print(grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST)
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        default.assert_called_once_with(scopes=None, default_scopes=None)
-        auth_metadata_plugin.assert_called_once_with(mock.sentinel.credentials, mock.sentinel.Request, default_host=default_host)
-    else:
-        default.assert_called_once_with(scopes=None)
-        auth_metadata_plugin.assert_called_once_with(mock.sentinel.credentials, mock.sentinel.Request)
+    default.assert_called_once_with(scopes=None, default_scopes=None)
+    auth_metadata_plugin.assert_called_once_with(mock.sentinel.credentials, mock.sentinel.Request, default_host=default_host)
 
     if grpc_helpers.HAS_GRPC_GCP:
         grpc_secure_channel.assert_called_once_with(target, composite_creds, None)
@@ -300,11 +289,7 @@
 
     grpc_helpers.create_channel(target, ssl_credentials=ssl_creds)
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        default.assert_called_once_with(scopes=None, default_scopes=None)
-    else:
-        default.assert_called_once_with(scopes=None)
+    default.assert_called_once_with(scopes=None, default_scopes=None)
 
     composite_creds_call.assert_called_once_with(ssl_creds, mock.ANY)
     composite_creds = composite_creds_call.return_value
@@ -331,11 +316,7 @@
 
     assert channel is grpc_secure_channel.return_value
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        default.assert_called_once_with(scopes=["one", "two"], default_scopes=None)
-    else:
-        default.assert_called_once_with(scopes=["one", "two"])
+    default.assert_called_once_with(scopes=["one", "two"], default_scopes=None)
 
     if grpc_helpers.HAS_GRPC_GCP:
         grpc_secure_channel.assert_called_once_with(target, composite_creds, None)
@@ -360,11 +341,7 @@
 
     assert channel is grpc_secure_channel.return_value
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        default.assert_called_once_with(scopes=None, default_scopes=["three", "four"])
-    else:
-        default.assert_called_once_with(scopes=["three", "four"])
+    default.assert_called_once_with(scopes=None, default_scopes=["three", "four"])
 
     if grpc_helpers.HAS_GRPC_GCP:
         grpc_secure_channel.assert_called_once_with(target, composite_creds, None)
@@ -392,11 +369,8 @@
 
     channel = grpc_helpers.create_channel(target, credentials=mock.sentinel.credentials)
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        auth_creds.assert_called_once_with(mock.sentinel.credentials, scopes=None, default_scopes=None)
-    else:
-        auth_creds.assert_called_once_with(mock.sentinel.credentials, scopes=None)
+    auth_creds.assert_called_once_with(mock.sentinel.credentials, scopes=None, default_scopes=None)
+
     assert channel is grpc_secure_channel.return_value
     if grpc_helpers.HAS_GRPC_GCP:
         grpc_secure_channel.assert_called_once_with(target, composite_creds, None)
@@ -418,11 +392,7 @@
         target, credentials=credentials, scopes=scopes
     )
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        credentials.with_scopes.assert_called_once_with(scopes, default_scopes=None)
-    else:
-        credentials.with_scopes.assert_called_once_with(scopes)
+    credentials.with_scopes.assert_called_once_with(scopes, default_scopes=None)
 
     assert channel is grpc_secure_channel.return_value
     if grpc_helpers.HAS_GRPC_GCP:
@@ -445,11 +415,7 @@
         target, credentials=credentials, default_scopes=default_scopes
     )
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        credentials.with_scopes.assert_called_once_with(scopes=None, default_scopes=default_scopes)
-    else:
-        credentials.with_scopes.assert_called_once_with(scopes=default_scopes)
+    credentials.with_scopes.assert_called_once_with(scopes=None, default_scopes=default_scopes)
 
     assert channel is grpc_secure_channel.return_value
     if grpc_helpers.HAS_GRPC_GCP:
@@ -498,11 +464,7 @@
         target, credentials_file=credentials_file
     )
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=None, default_scopes=None)
-    else:
-        google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=None)
+    google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=None, default_scopes=None)
 
     assert channel is grpc_secure_channel.return_value
     if grpc_helpers.HAS_GRPC_GCP:
@@ -529,11 +491,7 @@
         target, credentials_file=credentials_file, scopes=scopes
     )
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=scopes, default_scopes=None)
-    else:
-        google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=scopes)
+    google.auth.load_credentials_from_file.assert_called_once_with(credentials_file, scopes=scopes, default_scopes=None)
 
     assert channel is grpc_secure_channel.return_value
     if grpc_helpers.HAS_GRPC_GCP:
@@ -560,11 +518,7 @@
         target, credentials_file=credentials_file, default_scopes=default_scopes
     )
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        load_credentials_from_file.assert_called_once_with(credentials_file, scopes=None, default_scopes=default_scopes)
-    else:
-        load_credentials_from_file.assert_called_once_with(credentials_file, scopes=default_scopes)
+    load_credentials_from_file.assert_called_once_with(credentials_file, scopes=None, default_scopes=default_scopes)
 
     assert channel is grpc_secure_channel.return_value
     if grpc_helpers.HAS_GRPC_GCP:
@@ -587,11 +541,7 @@
     grpc_helpers.create_channel(target, credentials=credentials, scopes=scopes)
     grpc_gcp_secure_channel.assert_called()
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        credentials.with_scopes.assert_called_once_with(scopes, default_scopes=None)
-    else:
-        credentials.with_scopes.assert_called_once_with(scopes)
+    credentials.with_scopes.assert_called_once_with(scopes, default_scopes=None)
 
 
 @pytest.mark.skipif(grpc_helpers.HAS_GRPC_GCP, reason="grpc_gcp module not available")
@@ -606,11 +556,7 @@
     grpc_helpers.create_channel(target, credentials=credentials, scopes=scopes)
     grpc_secure_channel.assert_called()
 
-    # TODO: remove this if/else once google-auth >= 1.25.0 is required
-    if grpc_helpers._GOOGLE_AUTH_HAS_DEFAULT_SCOPES_AND_DEFAULT_HOST:
-        credentials.with_scopes.assert_called_once_with(scopes, default_scopes=None)
-    else:
-        credentials.with_scopes.assert_called_once_with(scopes)
+    credentials.with_scopes.assert_called_once_with(scopes, default_scopes=None)
 
 
 class TestChannelStub(object):