fix: run workflow with python 3.8

This is to work around jinja2 ImportError that prevents sphinx-build 3.2.1 to work.
See https://bugs.python.org/msg415979

Author: rffontenelle

.github/workflows/pythonpackage.yml

@@ -23,7 +23,7 @@ jobs:
         ref: ${{ env.BRANCH }}
     - uses: actions/setup-python@v2
       with:
-        python-version: 3.9
+        python-version: 3.8
     - run: sudo apt update
     - run: sudo apt install -y gettext
     - run: make tx-config