Add Python 3.8 to the test matrix

Co-Authored-By: Hugo van Kemenade <hugovk@users.noreply.github.com>

.github/workflows/python-tests.yml

@@ -14,6 +14,7 @@ jobs:
       # max-parallel: 5
       matrix:
         python-version:
+        - 3.8
         - 3.7
         - 3.6
         - 3.5