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/google/auth/_oauth2client.py b/google/auth/_oauth2client.py
index 95a9876..3512e1d 100644
--- a/google/auth/_oauth2client.py
+++ b/google/auth/_oauth2client.py
@@ -21,8 +21,6 @@
 
 from __future__ import absolute_import
 
-import six
-
 from google.auth import _helpers
 import google.auth.app_engine
 import google.auth.compute_engine
@@ -34,7 +32,7 @@
     import oauth2client.contrib.gce
     import oauth2client.service_account
 except ImportError as caught_exc:
-    six.raise_from(ImportError("oauth2client is not installed."), caught_exc)
+    raise ImportError("oauth2client is not installed.") from caught_exc
 
 try:
     import oauth2client.contrib.appengine  # pytype: disable=import-error
@@ -166,4 +164,4 @@
         return _CLASS_CONVERSION_MAP[credentials_class](credentials)
     except KeyError as caught_exc:
         new_exc = ValueError(_CONVERT_ERROR_TMPL.format(credentials_class))
-        six.raise_from(new_exc, caught_exc)
+        raise new_exc from caught_exc