Test Python 3.14 on Ubuntu and macOS on CI

The status of 3.14 is now effectively the same as the status of
3.13 when we started testing it in gitpython-developers#1990.

This doesn't enable it on Windows yet, for the same reason that we
still have not yet enabled regular CI tests of 3.13 on Windows. It
is hoped that 3.13 and 3.14 can be gotten fully working on Windows
(rather than just mostly working, we think) soon; these exclusions
are meant to be temporary.

Both the usual GIL interpreter and the free-threaded (nogil)
intepreters are tested. See the immediately preceding commit on the
tradeoffs involved in doing so.

Author: EliahKagan

.github/workflows/pythonpackage.yml

@@ -13,14 +13,18 @@ jobs:
     strategy:
       matrix:
         os-type: [ubuntu, macos, windows]
-        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13", "3.13t"]
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13", "3.13t", "3.14", "3.14t"]
         exclude:
         - os-type: macos
           python-version: "3.7"  # Not available for the ARM-based macOS runners.
         - os-type: windows
-          python-version: "3.13"  # FIXME: Fix and enable Python 3.13 on Windows (#1955).
+          python-version: "3.13"  # FIXME: Fix and enable Python 3.13 and 3.14 on Windows (#1955).
         - os-type: windows
           python-version: "3.13t"
+        - os-type: windows
+          python-version: "3.14"
+        - os-type: windows
+          python-version: "3.14t"
         include:
         - os-ver: latest
         - os-type: ubuntu