Downgrade to Python 3.8 since no wheel for 3.9

Not all wheels are available for Py3.9 yet, so CI stalls on installing
some packages (see also
docker-library/python#540)

Author: PGijsbers

.github/workflows/dist.yaml

@@ -10,7 +10,7 @@ jobs:
     - name: Setup Python
       uses: actions/setup-python@v2
       with:
-        python-version: 3.9
+        python-version: 3.8
     - name: build dist
       run: |
         python setup.py sdist