Revert "tox: Encode specific Python versions"
Revert "tox: Encode specific Python versions"

This reverts commit 89e321b7b5c88ab7cfab53103295774ca535f0df.

Reason for revert: The previous change require develper to install multiple spcific version python 
locally to run the test envs and it arbitrally pins lower constratins to the 
oldest python we support but lower contratis shoudl work with the newwst python we support too.
on fedora or other operating systems that ship with default python version that exceed our supported
versions test can be run with tox -e py38 if required.

similarly our updated constratins shoudl work with the older python.

Change-Id: I358ff8128746c9d0c584a03381e55817c4aace3f

openstack/nova,mahak/nova,openstack/nova,mahak/nova,mahak/nova,openstack/nova