Add py39 to tox

Author: einarf

tox.ini

@@ -8,6 +8,7 @@ envlist =
     py36
     py37
     py38
+    py39
 
 [testenv:pep8]
 usedevelop = false
@@ -22,6 +23,7 @@ basepython =
     py36: python3.6
     py37: python3.7
     py38: python3.8
+    py39: python3.9
 deps = -r{toxinidir}/tests/requirements.txt
 commands =
     pytest tests/