Update supported python versions

- Python 3.5 support was already removed from Tempest[1] so we can
  remove it.
- Add Python 3.8, 3.9 and 3.10 ecause these have been part of tested
  runtimes for a few cycles and are actually used in testing.

[1] b0a8057fbbf08ff072bfb6e836535f090fd6efc7

Change-Id: I39b822ef294f673adfb35e5d45672ad1f9347944
1 file changed