Add Python 3.12 to the Actions matrix

[OpenBagTwo]

Python 3.12 was released on Oct 2 :tada: :tada: :tada:

[OpenBagTwo]

FWIW, GSB v0.0.3-rc1 installs and runs fine on 3.12.

$ gsb -V
gsb, version 0.0.3-rc1
$ gsb test
============================================ test session starts ============================================
platform linux -- Python 3.12.0, pytest-7.4.2, pluggy-1.3.0
rootdir: /home/antar
plugins: cov-4.1.0
collected 190 items                                                                                         

test_backup.py ....................x...                                                               [ 12%]
test_delete.py ......x..................                                                              [ 25%]
test_export.py ................................................x                                      [ 51%]
test_history.py ..........................                                                            [ 65%]
test_init.py ....................................                                                     [ 84%]
test_rewind.py ..............................                                                         [100%]

====================================== 187 passed, 3 xfailed in 13.05s ======================================

Not exactly sure what the issue is with the test runner, but I'm also not motivated enough to spend energy looking into it. :shrug:

[OpenBagTwo]

May be blocked by https://github.com/libgit2/pygit2/issues/1240