Update python-app.yml

Author: tonybaloney

.github/workflows/python-app.yml

@@ -20,6 +20,7 @@ jobs:
         os: ["macos-11", ubuntu-20.04, "windows-latest"]
         python_version: ["3.7", "3.8", "3.9", "3.10", "3.11-dev"]
     steps:
+    - uses: actions/checkout@v2
     - name: Setup python
       uses: actions/setup-python@v2
       with: