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/aws.py b/google/auth/aws.py
index c2b521c..2f2a135 100644
--- a/google/auth/aws.py
+++ b/google/auth/aws.py
@@ -39,13 +39,12 @@
 
 import hashlib
 import hmac
+import http.client
 import io
 import json
 import os
 import re
-
-from six.moves import http_client
-from six.moves import urllib
+import urllib
 
 from google.auth import _helpers
 from google.auth import environment_vars
@@ -627,7 +626,7 @@
             else response.data
         )
 
-        if response.status != http_client.OK:
+        if response.status != http.client.OK:
             raise exceptions.RefreshError(
                 "Unable to retrieve AWS security credentials", response_body
             )
@@ -666,7 +665,7 @@
             else response.data
         )
 
-        if response.status != http_client.OK:
+        if response.status != http.client.OK:
             raise exceptions.RefreshError(
                 "Unable to retrieve AWS role name", response_body
             )