CI: Use Python 3.8 in PR jobs

Author: markgoddard

.github/workflows/pull-request.yml

@@ -9,9 +9,9 @@ jobs:
         matrix:
           include:
             - environment: pep8
-              python-version: 3.6 
+              python-version: 3.8
             - environment: releasenotes
-              python-version: 3.6 
+              python-version: 3.8
       name: Tox ${{ matrix.environment }} with Python ${{ matrix.python-version }}
       steps:
         - name: Github Checkout 🛎u