Go back to testing free-threaded interpreters only on GNU/Linux

This effectively reverts d1ab2e4. It doesn't look like any problems
arose, and contrary to my guess, the additional jobs do actually
make the checks that we intend to be blocking for PRs take longer,
even after all non-macOS checks have completed.

Author: EliahKagan

.github/workflows/pythonpackage.yml

@@ -17,6 +17,10 @@ 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: macos
+          python-version: "3.14t"
         - os-type: windows
           python-version: "3.13"  # FIXME: Fix and enable Python 3.13 and 3.14 on Windows (#1955).
         - os-type: windows