[skip travis] select python 3.8 in workflow file

.github/workflows/build.yml

@@ -20,7 +20,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v2
       with:
-          python-version: '3.x'
+          python-version: '3.8'
     - name: Install packages
       run: sudo apt-get -y install zip pandoc man
     - name: Bump version