ci: bump actions/setup-python to v5

I noticed looking at recent workflow runs that there's a number of
warnings where the cache has failed to save/restore.

Apparently old versions of the actions cache service are now deprecated
(https://github.com/actions/setup-python/pull/1007).

Bumping actions/setup-python to v5 should hopefully fix the caching and
help speed up CI runs!

.github/workflows/pythonpackage.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Install Poetry
         run: pipx install poetry==1.7.1
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v4.7.1
+        uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
           cache: "poetry"