Exclude unavailable Python versions for M1 runner

Some versions of Python do not currently have builds available via
actions/setup-python for the Apple Silicon M1 (64-bit ARM) runner
used for macos-14. This excludes those.

- Python 3.7 is EoL and builds for it are no likely to be provided
  for newly available platforms on GitHub Actions.

- Python 3.8 and 3.9 are still supported by the Python Software
  Foundation. Builds for them are not currently avaialable on the
  GHA M1 runners, but it appears this may not be intentional. See
  actions/setup-python#808. If those
  versions become available, then they can be reenabled.

- Later versions of Python are available.

.github/workflows/pythonpackage.yml

@@ -15,6 +15,13 @@ jobs:
       matrix:
         os: ["ubuntu-latest", "macos-13", "macos-14", "windows-latest"]
         python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
+        exclude:
+        - os: "macos-14"
+          python-version: "3.7"
+        - os: "macos-14"
+          python-version: "3.8"
+        - os: "macos-14"
+          python-version: "3.9"
         include:
         - experimental: false