setup.py: add basic packaging files

This is needed to use tox, and tox lets us test multiple Python
versions easily.

Change-Id: I813c418a8f7109294a4adb9f6b21be459cbeca70
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/247173
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>