[Doc] Fix python version in unit tests document

This commit fixes the python version which is supported officially. We
dropped Python 3.6 in the past commit[1] already and Python 2.7 is not
no longer supported. So we should be able to drop and replace the tox
job in the document.

[1] Ie844887a798a006794729e733748ceaa5e06d2db

Change-Id: If2998dd984aeb7d93bd49659d08a7212e16069c0
diff --git a/doc/source/overview.rst b/doc/source/overview.rst
index 2eaf72f..315255d 100644
--- a/doc/source/overview.rst
+++ b/doc/source/overview.rst
@@ -207,21 +207,21 @@
 is ``test_path=./tempest/test_discover`` which will only run test discover on the
 Tempest suite.
 
-Alternatively, there are the py27 and py36 tox jobs which will run the unit
-tests with the corresponding version of python.
+Alternatively, there is the py39 tox job which will run the unit tests with
+the corresponding version of python.
 
 One common activity is to just run a single test, you can do this with tox
-simply by specifying to just run py27 or py36 tests against a single test::
+simply by specifying to just run py39 tests against a single test::
 
-    $ tox -e py36 -- -n tempest.tests.test_microversions.TestMicroversionsTestsClass.test_config_version_none_23
+    $ tox -e py39 -- -n tempest.tests.test_microversions.TestMicroversionsTestsClass.test_config_version_none_23
 
 Or all tests in the test_microversions.py file::
 
-    $ tox -e py36 -- -n tempest.tests.test_microversions
+    $ tox -e py39 -- -n tempest.tests.test_microversions
 
 You may also use regular expressions to run any matching tests::
 
-    $ tox -e py36 -- test_microversions
+    $ tox -e py39 -- test_microversions
 
 Additionally, when running a single test, or test-file, the ``-n/--no-discover``
 argument is no longer required, however it may perform faster if included.