Test on free-threaded Python

See gitpython-developers#2005. Right now, this does not limit by operating system, but
that is just to verify that there are no OS-specific 3.13t problems
we should know about right now; once that is verified, the macOS
and Windows jobs will be removed (excluded) for the time being.

The 3.13t jobs added here use `Quansight-Labs/setup-python`, not
`actions/setup-python`. The latter also has the ability to use
3.13t since actions/python-versions#319
and actions/setup-python#973 (see also
actions/setup-python#771), but no version
tag includes this feature yet. It can be used by using `@main` or
`@...` where `...` is an OID. The former would risk pulling in
other untested features we're no trying to test with, while the
latter would not be easy to upgrade automatically as what we have
now (we would be deliberately keeping a hash not at any tag that is
already not the latest hash on any branch). In contrast, the
`Quansight-Labs/setup-python` fork adds this feature while staying
up to date with others. When `actions/setup-python` has a release
(stable or prerelease) with this feature, we can switch to it.

This could probably be done with less code duplication by using a
matrix variable for the action to use. Instead, the "Set up Python"
step is split in two, with opposite `if` conditions, so that each
is capable of being recognized and upgraded by Dependabot if a new
major version is released (in case this ends up remaining in place
longer than expected).

Author: EliahKagan

.github/workflows/pythonpackage.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         os-type: [ubuntu, macos, windows]
-        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13", "3.13t"]
         exclude:
         - os-type: macos
           python-version: "3.7"  # Not available for the ARM-based macOS runners.
@@ -40,11 +40,20 @@ jobs:
         fetch-depth: 0
 
     - name: Set up Python ${{ matrix.python-version }}
+      if: |-
+        !endsWith(matrix.python-version, 't')
       uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
         allow-prereleases: ${{ matrix.experimental }}
 
+    - name: Set up Python ${{ matrix.python-version }} (free-threaded)
+      if: endsWith(matrix.python-version, 't')
+      uses: Quansight-Labs/setup-python@v5
+      with:
+        python-version: ${{ matrix.python-version }}
+        allow-prereleases: ${{ matrix.experimental }}
+
     - name: Set up WSL (Windows)
       if: matrix.os-type == 'windows'
       uses: Vampire/setup-wsl@v5.0.0