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_iam.py b/tests/test_iam.py
index 382713b..30ce227 100644
--- a/tests/test_iam.py
+++ b/tests/test_iam.py
@@ -14,11 +14,11 @@
 
 import base64
 import datetime
+import http.client
 import json
 
 import mock
 import pytest
-from six.moves import http_client
 
 from google.auth import _helpers
 from google.auth import exceptions
@@ -81,7 +81,7 @@
     def test_sign_bytes(self):
         signature = b"DEADBEEF"
         encoded_signature = base64.b64encode(signature).decode("utf-8")
-        request = make_request(http_client.OK, data={"signedBlob": encoded_signature})
+        request = make_request(http.client.OK, data={"signedBlob": encoded_signature})
         credentials = make_credentials()
 
         signer = iam.Signer(request, credentials, mock.sentinel.service_account_email)
@@ -93,7 +93,7 @@
         assert kwargs["headers"]["Content-Type"] == "application/json"
 
     def test_sign_bytes_failure(self):
-        request = make_request(http_client.UNAUTHORIZED)
+        request = make_request(http.client.UNAUTHORIZED)
         credentials = make_credentials()
 
         signer = iam.Signer(request, credentials, mock.sentinel.service_account_email)