[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
1 file changed