Test on Python 3.9 as well

Author: altendky

.github/workflows/ci.yml

@@ -21,6 +21,7 @@ jobs:
           - py36,report,codecov
           - py37,report,codecov
           - py38,report,codecov
+          - py39,report,codecov
           - pypy3,report,codecov
           - doc
           - check