This is analogous to the gitdb test workflow and `setup.py` updates
in gitpython-developers/gitdb#114.

1. Testing 3.7 on 22.04 rather than 24.04 should fix the problem
   where it fails because Python 3.7 is not available.

2. Adding Ubuntu 3.13 to CI may help reveal if there are
   3.13-specific problems with smmap.

3. smmap seems to be working on Python 3.13 (which is a stable
   Python release) and there are no specific expected problems with
   it, so this adds it to the list of supported releases.

In particular, this change, due to (1), fixes the current CI
failure for smmap observed in f31bfa3.

Test Python 3.7 on Ubuntu 22.04, and add Ubuntu 3.13

See gitpython-developers/gitdb#116.

Add SECURITY.md, referencing the GitPython one