🔧 Try working around a weird install cache issue.

Somehow github action keeps using an obsolete version of sources for oauth_dropins.

.github/workflows/automatic-tests.yml

@@ -21,6 +21,7 @@ jobs:
           python -m pip install --upgrade pip
           pip install --upgrade pytest
           pip install --upgrade -r requirements-tests.txt
+          pip install webapp2 # workaround a weird install cache issue
       - name: Lint with flake8
         run: |
           # stop the build if there are Python syntax errors or undefined names