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/credentials.py b/google/auth/credentials.py
index 7d3c798..6356f54 100644
--- a/google/auth/credentials.py
+++ b/google/auth/credentials.py
@@ -17,13 +17,10 @@
 
 import abc
 
-import six
-
 from google.auth import _helpers
 
 
-@six.add_metaclass(abc.ABCMeta)
-class Credentials(object):
+class Credentials(object, metaclass=abc.ABCMeta):
     """Base class for all credentials.
 
     All credentials have a :attr:`token` that is used for authentication and
@@ -187,8 +184,7 @@
         """Anonymous credentials do nothing to the request."""
 
 
-@six.add_metaclass(abc.ABCMeta)
-class ReadOnlyScoped(object):
+class ReadOnlyScoped(object, metaclass=abc.ABCMeta):
     """Interface for credentials whose scopes can be queried.
 
     OAuth 2.0-based credentials allow limiting access using scopes as described
@@ -329,8 +325,7 @@
         return credentials
 
 
-@six.add_metaclass(abc.ABCMeta)
-class Signing(object):
+class Signing(object, metaclass=abc.ABCMeta):
     """Interface for credentials that can cryptographically sign messages."""
 
     @abc.abstractmethod