docs: omit mention of Python 2.7 in 'CONTRIBUTING.rst' (#1127)

Closes #1126

Author: tseaver

synthtool/gcp/templates/python_library/CONTRIBUTING.rst

@@ -69,7 +69,6 @@ We use `nox <https://nox.readthedocs.io/en/latest/>`__ to instrument our tests.
 
 - To test your changes, run unit tests with ``nox``::
 
-    $ nox -s unit-2.7
     $ nox -s unit-3.8
     $ ...
 
@@ -144,17 +143,15 @@ Running System Tests
 
    # Run all system tests
    $ nox -s system-3.8
-   $ nox -s system-2.7
 
    # 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
-      Python 3.8. For expediency, we do not run them in older versions
-      of Python 3.
+      System tests are only configured to run under Python 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