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/crypt/base.py b/google/auth/crypt/base.py
index c98d5bf..0bda9c3 100644
--- a/google/auth/crypt/base.py
+++ b/google/auth/crypt/base.py
@@ -18,15 +18,12 @@
 import io
 import json
 
-import six
-
 
 _JSON_FILE_PRIVATE_KEY = "private_key"
 _JSON_FILE_PRIVATE_KEY_ID = "private_key_id"
 
 
-@six.add_metaclass(abc.ABCMeta)
-class Verifier(object):
+class Verifier(object, metaclass=abc.ABCMeta):
     """Abstract base class for crytographic signature verifiers."""
 
     @abc.abstractmethod
@@ -46,8 +43,7 @@
         raise NotImplementedError("Verify must be implemented")
 
 
-@six.add_metaclass(abc.ABCMeta)
-class Signer(object):
+class Signer(object, metaclass=abc.ABCMeta):
     """Abstract base class for cryptographic signers."""
 
     @abc.abstractproperty
@@ -70,8 +66,7 @@
         raise NotImplementedError("Sign must be implemented")
 
 
-@six.add_metaclass(abc.ABCMeta)
-class FromServiceAccountMixin(object):
+class FromServiceAccountMixin(object, metaclass=abc.ABCMeta):
     """Mix-in to enable factory constructors for a Signer."""
 
     @abc.abstractmethod