chore(python): simplify nox steps in CONTRIBUTING.rst (#2)

Source-Link: googleapis/synthtool@26558ba
Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:99d90d097e4a4710cc8658ee0b5b963f4426d0e424819787c3ac1405c9a26719

.github/.OwlBot.lock.yaml

@@ -1,17 +1,3 @@
-# Copyright 2021 Google LLC
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-# http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
 docker:
   image: gcr.io/repo-automation-bots/owlbot-python:latest
-  digest: sha256:c66ba3c8d7bc8566f47df841f98cd0097b28fff0b1864c86f5817f4c8c3e8600
+  digest: sha256:99d90d097e4a4710cc8658ee0b5b963f4426d0e424819787c3ac1405c9a26719

CONTRIBUTING.rst

@@ -68,14 +68,9 @@ Using ``nox``
 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
 
-    $ nox -s unit-3.6
-    $ nox -s unit-3.7
-    $ nox -s unit-3.8
-    $ nox -s unit-3.9
-
-- Args to pytest can be passed through the nox command separated by a `--`. For
-  example, to run a single test::
+- To run a single unit test::
 
     $ nox -s unit-3.9 -- -k <name of test>
 
@@ -145,7 +140,7 @@ Running System Tests
 - To run system tests, you can execute::
 
    # Run all system tests
-   $ nox -s system-3.8
+   $ nox -s system
 
    # Run a single system test
    $ nox -s system-3.8 -- -k <name of test>