tox: enable python 3.5 & 3.9 testing

We still support Python 3.5, so make sure it keeps working.

Change-Id: I150158a656b26de6d733316a68a2cbb8b5b99716
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/299625


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>