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__oauth2client.py b/tests/test__oauth2client.py
index 6b1112b..aa06ece 100644
--- a/tests/test__oauth2client.py
+++ b/tests/test__oauth2client.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import datetime
+import importlib
 import os
 import sys
 
@@ -21,7 +22,6 @@
 import oauth2client.contrib.gce
 import oauth2client.service_account
 import pytest
-from six.moves import reload_module
 
 from google.auth import _oauth2client
 
@@ -152,19 +152,19 @@
 @pytest.fixture
 def reset__oauth2client_module():
     """Reloads the _oauth2client module after a test."""
-    reload_module(_oauth2client)
+    importlib.reload(_oauth2client)
 
 
 def test_import_has_app_engine(
     mock_oauth2client_gae_imports, reset__oauth2client_module
 ):
-    reload_module(_oauth2client)
+    importlib.reload(_oauth2client)
     assert _oauth2client._HAS_APPENGINE
 
 
 def test_import_without_oauth2client(monkeypatch, reset__oauth2client_module):
     monkeypatch.setitem(sys.modules, "oauth2client", None)
     with pytest.raises(ImportError) as excinfo:
-        reload_module(_oauth2client)
+        importlib.reload(_oauth2client)
 
     assert excinfo.match("oauth2client")