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/id_token.py b/google/oauth2/id_token.py
index 540ccd1..25492ca 100644
--- a/google/oauth2/id_token.py
+++ b/google/oauth2/id_token.py
@@ -55,12 +55,10 @@
 .. _CacheControl: https://cachecontrol.readthedocs.io
 """
 
+import http.client
 import json
 import os
 
-import six
-from six.moves import http_client
-
 from google.auth import environment_vars
 from google.auth import exceptions
 from google.auth import jwt
@@ -97,7 +95,7 @@
     """
     response = request(certs_url, method="GET")
 
-    if response.status != http_client.OK:
+    if response.status != http.client.OK:
         raise exceptions.TransportError(
             "Could not fetch certificates at {}".format(certs_url)
         )
@@ -242,10 +240,10 @@
                 "GOOGLE_APPLICATION_CREDENTIALS is not valid service account credentials.",
                 caught_exc,
             )
-            six.raise_from(new_exc, caught_exc)
+            raise new_exc from caught_exc
 
-    # 2. Try to fetch ID token from metada server if it exists. The code works for GAE and
-    # Cloud Run metadata server as well.
+    # 2. Try to fetch ID token from metada server if it exists. The code
+    # works for GAE and Cloud Run metadata server as well.
     try:
         from google.auth import compute_engine
         from google.auth.compute_engine import _metadata