Fix tox -e build by pinning Python version (#51)

`python -m build` was giving issues with Python 3.12.1, so pinned to version 3.9

.github/workflows/package.yml

@@ -16,9 +16,9 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python
-      uses: actions/setup-python@v2
+      uses: actions/setup-python@v4
       with:
-        python-version: '3.x'
+        python-version: '3.9'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip