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/external_account.py b/google/auth/external_account.py
index 1f3034a..24b93b4 100644
--- a/google/auth/external_account.py
+++ b/google/auth/external_account.py
@@ -33,8 +33,6 @@
 import json
 import re
 
-import six
-
 from google.auth import _helpers
 from google.auth import credentials
 from google.auth import exceptions
@@ -52,8 +50,9 @@
 _CLOUD_RESOURCE_MANAGER = "https://cloudresourcemanager.googleapis.com/v1/projects/"
 
 
-@six.add_metaclass(abc.ABCMeta)
-class Credentials(credentials.Scoped, credentials.CredentialsWithQuotaProject):
+class Credentials(
+    credentials.Scoped, credentials.CredentialsWithQuotaProject, metaclass=abc.ABCMeta
+):
     """Base class for all external account credentials.
 
     This is used to instantiate Credentials for exchanging external account