Add classifiers for specific versions of Python

These are used by automated tools, such as caniusepython3
(https://github.com/brettcannon/caniusepython3) to check on what
version of Python the Nova client works.

Change-Id: Ie2a652a8c4797b1e1a69c91673e6009f36772667
Merge "Add classifiers for specific versions of Python"