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

    Source-Link: googleapis/synthtool@b91f129
    Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:b6169fc6a5207b11800a7c002d0c5c2bc6d82697185ca12e666f44031468cfcd
    Closes #1126

.github/.OwlBot.lock.yaml

@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/repo-automation-bots/owlbot-python:latest
-  digest: sha256:58c7342b0bccf85028100adaa3d856cb4a871c22ca9c01960d996e66c40548ce
+  digest: sha256:b6169fc6a5207b11800a7c002d0c5c2bc6d82697185ca12e666f44031468cfcd

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