fix!: drop support for Python 2.7 (#778)

Drop use of 'six' wrapper library.

Drop 'u"' prefixes.

Drop support for app_engine 'classic' mode (Python 2.7-only).

Release-As: 2.0.0b1

Closes #777.
diff --git a/tests/test_impersonated_credentials.py b/tests/test_impersonated_credentials.py
index 90de704..126c4c3 100644
--- a/tests/test_impersonated_credentials.py
+++ b/tests/test_impersonated_credentials.py
@@ -13,12 +13,12 @@
 # limitations under the License.
 
 import datetime
+import http.client
 import json
 import os
 
 import mock
 import pytest
-from six.moves import http_client
 
 from google.auth import _helpers
 from google.auth import crypt
@@ -79,7 +79,7 @@
         "google.auth.transport.requests.AuthorizedSession.request", autospec=True
     ) as auth_session:
         data = {"keyId": "1", "signedBlob": "c2lnbmF0dXJl"}
-        auth_session.return_value = MockResponse(data, http_client.OK)
+        auth_session.return_value = MockResponse(data, http.client.OK)
         yield auth_session
 
 
@@ -89,7 +89,7 @@
         "google.auth.transport.requests.AuthorizedSession.request", autospec=True
     ) as auth_session:
         data = {"token": ID_TOKEN_DATA}
-        auth_session.return_value = MockResponse(data, http_client.OK)
+        auth_session.return_value = MockResponse(data, http.client.OK)
         yield auth_session
 
 
@@ -141,7 +141,7 @@
     def make_request(
         self,
         data,
-        status=http_client.OK,
+        status=http.client.OK,
         headers=None,
         side_effect=None,
         use_data_bytes=True,
@@ -169,7 +169,7 @@
 
         request = self.make_request(
             data=json.dumps(response_body),
-            status=http_client.OK,
+            status=http.client.OK,
             use_data_bytes=use_data_bytes,
         )
 
@@ -194,7 +194,7 @@
 
         request = self.make_request(
             data=json.dumps(response_body),
-            status=http_client.OK,
+            status=http.client.OK,
             use_data_bytes=use_data_bytes,
         )
 
@@ -229,7 +229,7 @@
             ).isoformat("T") + "Z"
             response_body = {"accessToken": "token", "expireTime": expire_time}
             request = self.make_request(
-                data=json.dumps(response_body), status=http_client.OK
+                data=json.dumps(response_body), status=http.client.OK
             )
 
             credentials.refresh(request)
@@ -254,7 +254,7 @@
         response_body = {"accessToken": token, "expireTime": expire_time}
 
         request = self.make_request(
-            data=json.dumps(response_body), status=http_client.OK
+            data=json.dumps(response_body), status=http.client.OK
         )
 
         with pytest.raises(exceptions.RefreshError) as excinfo:
@@ -277,7 +277,7 @@
         }
 
         request = self.make_request(
-            data=json.dumps(response_body), status=http_client.UNAUTHORIZED
+            data=json.dumps(response_body), status=http.client.UNAUTHORIZED
         )
 
         with pytest.raises(exceptions.RefreshError) as excinfo:
@@ -294,7 +294,7 @@
         response_body = {}
 
         request = self.make_request(
-            data=json.dumps(response_body), status=http_client.HTTPException
+            data=json.dumps(response_body), status=http.client.HTTPException
         )
 
         with pytest.raises(exceptions.RefreshError) as excinfo:
@@ -331,7 +331,7 @@
         token_response_body = {"accessToken": token, "expireTime": expire_time}
 
         response = mock.create_autospec(transport.Response, instance=False)
-        response.status = http_client.OK
+        response.status = http.client.OK
         response.data = _helpers.to_bytes(json.dumps(token_response_body))
 
         request = mock.create_autospec(transport.Request, instance=False)
@@ -369,7 +369,7 @@
 
         request = self.make_request(
             data=json.dumps(response_body),
-            status=http_client.OK,
+            status=http.client.OK,
             use_data_bytes=use_data_bytes,
         )
 
@@ -394,7 +394,7 @@
         response_body = {"accessToken": token, "expireTime": expire_time}
 
         request = self.make_request(
-            data=json.dumps(response_body), status=http_client.OK
+            data=json.dumps(response_body), status=http.client.OK
         )
 
         credentials.refresh(request)
@@ -423,7 +423,7 @@
         response_body = {"accessToken": token, "expireTime": expire_time}
 
         request = self.make_request(
-            data=json.dumps(response_body), status=http_client.OK
+            data=json.dumps(response_body), status=http.client.OK
         )
 
         credentials.refresh(request)
@@ -453,7 +453,7 @@
         response_body = {"accessToken": token, "expireTime": expire_time}
 
         request = self.make_request(
-            data=json.dumps(response_body), status=http_client.OK
+            data=json.dumps(response_body), status=http.client.OK
         )
 
         credentials.refresh(request)
@@ -494,7 +494,7 @@
         response_body = {"accessToken": token, "expireTime": expire_time}
 
         request = self.make_request(
-            data=json.dumps(response_body), status=http_client.OK
+            data=json.dumps(response_body), status=http.client.OK
         )
 
         credentials.refresh(request)
@@ -523,7 +523,7 @@
         response_body = {"accessToken": token, "expireTime": expire_time}
 
         request = self.make_request(
-            data=json.dumps(response_body), status=http_client.OK
+            data=json.dumps(response_body), status=http.client.OK
         )
 
         credentials.refresh(request)