ci: pin to python 3.8.7

Author: boriel

.github/workflows/python-app.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Set up Python 3.8
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.8.7
     - name: Caches pip
       uses: actions/cache@v1
       with: