This is analogous to the gitdb test workflow and `setup.py` were
updated inhttps://github.com/gitpython-developers/gitdb/pull/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.

This is analogous to the gitdb test workflow and `setup.py` were
updated 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.

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.

This is analogous to the 3.7-related CI change in gitdb that was
part of gitpython-developers/gitdb#114, as
to part of gitpython-developers/smmap#58.

Since some tests are not yet passing on 3.13, this does not add
3.13 to CI, nor to the documentation of supported versions in
`setup.py`. Note that the list there is not enforced; GitPython can
already be installed on Python 3.13 and probably *mostly* works.

(See gitpython-developers#1955
for details on other changes that should be made to fully support
running GitPython on Python 3.13.)