fix!: drop support for Python 2.7 / 3.5 (#212)

Drop 'six' module

Drop 'u"' prefixes for text

Remove other Python 2.7 workarounds

Drop use of 'pytz'

Dxpand range to allow 'google-auth' 2.x versions

Remove 'general_helpers.wraps': except for a backward-compatibility
import, 'functools.wraps' does everything wee need on Python >= 3.6.

Remove 'packaging' dependency

Release-As: 2.0.0b1

Closes #74.

Closes #215.
diff --git a/README.rst b/README.rst
index 244043e..d94f3e8 100644
--- a/README.rst
+++ b/README.rst
@@ -1,7 +1,7 @@
 Core Library for Google Client Libraries
 ========================================
 
-|pypi| |versions| 
+|pypi| |versions|
 
 This library is not meant to stand-alone. Instead it defines
 common helpers used by all Google API clients. For more information, see the
@@ -16,8 +16,13 @@
 
 Supported Python Versions
 -------------------------
-Python >= 3.5
+Python >= 3.6
 
-Deprecated Python Versions
---------------------------
-Python == 2.7. Python 2.7 support will be removed on January 1, 2020.
+
+Unsupported Python Versions
+---------------------------
+
+Python == 2.7, Python == 3.5.
+
+The last version of this library compatible with Python 2.7 and 3.5 is
+`google-api_core==1.31.1`.