macOS: fix workflow by invoking python3 not python

.github/workflows/macos-package.yml

@@ -21,8 +21,8 @@ jobs:
         echo "/Library/Frameworks/Python.framework/Versions/${PYVER_SHORT}/bin" >> $GITHUB_PATH
     - name: Install dependencies
       run: |
-        python -m pip install --upgrade pip
-        python -m pip install --no-binary=Pillow \
+        python3 -m pip install --upgrade pip
+        python3 -m pip install --no-binary=Pillow \
           Pillow attrs configobj py2app pycotap pysol-cards random2 setuptools six
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
         brew install create-dmg
@@ -36,7 +36,7 @@ jobs:
         make rules mo
     - name: Create the executable .app
       run: |
-        PYTHONPATH="." python setup_osx.py py2app
+        PYTHONPATH="." python3 setup_osx.py py2app
         zip -q -r -X PySolFC-app.zip dist
     - name: Make the .dmg for easy installation
       run: