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/oauth2/sts.py b/google/oauth2/sts.py
index ae3c014..9f2d68a 100644
--- a/google/oauth2/sts.py
+++ b/google/oauth2/sts.py
@@ -31,10 +31,9 @@
 .. _rfc8693 section 2.2.1: https://tools.ietf.org/html/rfc8693#section-2.2.1
 """
 
+import http.client
 import json
-
-from six.moves import http_client
-from six.moves import urllib
+import urllib
 
 from google.oauth2 import utils
 
@@ -146,7 +145,7 @@
         )
 
         # If non-200 response received, translate to OAuthError exception.
-        if response.status != http_client.OK:
+        if response.status != http.client.OK:
             utils.handle_error_response(response_body)
 
         response_data = json.loads(response_body)