Use PyInstaller 3.5 for macOS builds

PyInstaller 3.6 is causing runtime errors on start.

.github/workflows/package-macos.yml

@@ -49,6 +49,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip setuptools wheel
         pip install -r requirements-build.txt
+        pip install PyInstaller==3.5
         pip install -r requirements-macos.txt
     - name: Run tests
       run: |