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/iam.py b/google/auth/iam.py
index 5d63dc5..277f4b7 100644
--- a/google/auth/iam.py
+++ b/google/auth/iam.py
@@ -20,10 +20,9 @@
 """
 
 import base64
+import http.client
 import json
 
-from six.moves import http_client
-
 from google.auth import _helpers
 from google.auth import crypt
 from google.auth import exceptions
@@ -77,7 +76,7 @@
         self._credentials.before_request(self._request, method, url, headers)
         response = self._request(url=url, method=method, body=body, headers=headers)
 
-        if response.status != http_client.OK:
+        if response.status != http.client.OK:
             raise exceptions.TransportError(
                 "Error calling the IAM signBlob API: {}".format(response.data)
             )