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/identity_pool.py b/google/auth/identity_pool.py
index 5362199..c331e09 100644
--- a/google/auth/identity_pool.py
+++ b/google/auth/identity_pool.py
@@ -33,11 +33,7 @@
 access tokens.
 """
 
-try:
-    from collections.abc import Mapping
-# Python 2.7 compatibility
-except ImportError:  # pragma: NO COVER
-    from collections import Mapping
+from collections.abc import Mapping
 import io
 import json
 import os