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/CONTRIBUTING.rst b/CONTRIBUTING.rst
index c7860ad..358404f 100644
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -1,4 +1,3 @@
-.. Generated by synthtool. DO NOT EDIT!
 ############
 Contributing
 ############
@@ -22,7 +21,7 @@
   documentation.
 
 - The feature must work fully on the following CPython versions:
-  2.7, 3.6, 3.7, 3.8 and 3.9 on both UNIX and Windows.
+  3.6, 3.7, 3.8 and 3.9 on both UNIX and Windows.
 
 - The feature must not add unnecessary dependencies (where
   "unnecessary" is of course subjective, but new dependencies should
@@ -77,8 +76,8 @@
 
   .. note::
 
-    The unit tests and system tests are described in the
-    ``noxfile.py`` files in each directory.
+    The unit tests tests are described in the ``noxfile.py`` files
+    in each directory.
 
 .. nox: https://pypi.org/project/nox/
 
@@ -133,29 +132,6 @@
   "Function-Under-Test"), which is PEP8-incompliant, but more readable.
   Some also use a local variable, ``MUT`` (short for "Module-Under-Test").
 
-********************
-Running System Tests
-********************
-
-- To run system tests, you can execute::
-
-   # Run all system tests
-   $ nox -s system
-
-   # Run a single system test
-   $ nox -s system-3.8 -- -k <name of test>
-
-
-  .. note::
-
-      System tests are only configured to run under Python 2.7 and 3.8.
-      For expediency, we do not run them in older versions of Python 3.
-
-  This alone will not run the tests. You'll need to change some local
-  auth settings and change some configuration in your project to
-  run all the tests.
-
-- System tests will be run against an actual project. You should use local credentials from gcloud when possible. See `Best practices for application authentication <https://cloud.google.com/docs/authentication/best-practices-applications#local_development_and_testing_with_the>`__. Some tests require a service account. For those tests see `Authenticating as a service account <https://cloud.google.com/docs/authentication/production>`__.
 
 *************
 Test Coverage
@@ -221,13 +197,11 @@
 
 We support:
 
--  `Python 2.7`_
 -  `Python 3.6`_
 -  `Python 3.7`_
 -  `Python 3.8`_
 -  `Python 3.9`_
 
-.. _Python 2.7: https://docs.python.org/2.7/
 .. _Python 3.6: https://docs.python.org/3.6/
 .. _Python 3.7: https://docs.python.org/3.7/
 .. _Python 3.8: https://docs.python.org/3.8/
@@ -239,7 +213,7 @@
 .. _config: https://github.com/googleapis/python-api-core/blob/master/noxfile.py
 
 
-We also explicitly decided to support Python 3 beginning with version 2.7.
+We also explicitly decided to support Python 3 beginning with version 3.6.
 Reasons for this include:
 
 -  Encouraging use of newest versions of Python 3