docs(python): document adding Python 3.9 support, dropping 3.5 support (#880)

Closes #787

Author: tseaver

synthtool/gcp/templates/python_library/CONTRIBUTING.rst

@@ -21,8 +21,8 @@ In order to add a feature:
 - The feature must be documented in both the API and narrative
  documentation.
 
-- The feature must work fully on the following CPython versions: 2.7,
- 3.5, 3.6, 3.7 and 3.8 on both UNIX and Windows.
+- The feature must work fully on the following CPython versions:
+ 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
@@ -202,25 +202,24 @@ Supported Python Versions
 
 We support:
 
-- `Python 3.5`_
 - `Python 3.6`_
 - `Python 3.7`_
 - `Python 3.8`_
+- `Python 3.9`_
 
-.. _Python 3.5: https://docs.python.org/3.5/
 .. _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/
+.. _Python 3.9: https://docs.python.org/3.9/
 
 
 Supported versions can be found in our ``noxfile.py`` `config`_.
 
 .. _config: https://github.com/{{ metadata['repo']['repo'] }}/blob/master/noxfile.py
 
-Python 2.7 support is deprecated. All code changes should maintain Python 2.7 compatibility until January 1, 2020.
 
 We also explicitly decided to support Python 3 beginning with version
-3.5. Reasons for this include:
+3.6. Reasons for this include:
 
 - Encouraging use of newest versions of Python 3
 - Taking the lead of `prominent`_ open-source `projects`_