Only test free-threaded Python on Linux

For now, this omits macOS and Windows from the 3.13t ("threaded")
tests.

The plan in gitpython-developers#2005 is to start without them, and no OS-specific
problems have been identified so far. In particular, in the
previous commit that adds 3.13t without excluding any operating
systems, all tests in the macOS job passed as expected, and the
Windows job had the same failure with the same message as in gitpython-developers#1955,
with no XFAIL changed to XPASS (which, if present, would suggest
GC differences meriting further exploration of 3.13t on Windows).

Author: EliahKagan

Diff for: .github/workflows/pythonpackage.yml

@@ -17,8 +17,12 @@ jobs:
         exclude:
         - os-type: macos
           python-version: "3.7"  # Not available for the ARM-based macOS runners.
+        - os-type: macos
+          python-version: "3.13t"
         - os-type: windows
           python-version: "3.13"  # FIXME: Fix and enable Python 3.13 on Windows (#1955).
+        - os-type: windows
+          python-version: "3.13t"
         include:
         - os-ver: latest
         - os-type: ubuntu