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/impersonated_credentials.py b/google/auth/impersonated_credentials.py
index b8a6c49..2704bfd 100644
--- a/google/auth/impersonated_credentials.py
+++ b/google/auth/impersonated_credentials.py
@@ -28,11 +28,9 @@
 import base64
 import copy
 from datetime import datetime
+import http.client
 import json
 
-import six
-from six.moves import http_client
-
 from google.auth import _helpers
 from google.auth import credentials
 from google.auth import exceptions
@@ -100,7 +98,7 @@
         else response.data
     )
 
-    if response.status != http_client.OK:
+    if response.status != http.client.OK:
         exceptions.RefreshError(_REFRESH_ERROR, response_body)
 
     try:
@@ -117,7 +115,7 @@
             ),
             response_body,
         )
-        six.raise_from(new_exc, caught_exc)
+        raise new_exc from caught_exc
 
 
 class Credentials(credentials.CredentialsWithQuotaProject, credentials.Signing):