Add CI test job for no-GIL ("threaded") Python 3.13

See gitpython-developers/GitPython#2005.

The rationale is that, while this is probably less important to do
in gitdb and smmap, any failure that arises for this in GitPython
would likely raise the question of whether a correspond problem has
begun to occur in gitdb and smmap (both of which provide helpers
used in GitPython even when the in-memory object database is not
used).

Author: EliahKagan

.github/workflows/pythonpackage.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
+        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12", "3.13", "3.13t"]
         os: [ubuntu-latest]
         experimental: [false]
         include: