CI: Test on Python 3.9

Author: effigies

.github/workflows/pythonpackage.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7, 3.8]
+        python-version: [3.7, 3.8, 3.9]
         pip-flags: ['', '--editable']
         pydra:
         - 'pydra'