Add Python 3.9 into tox.ini

Author: altendky

tox.ini

@@ -5,7 +5,7 @@ envlist =
     clean,
     check,
     doc,
-    {py36,py37,py38,pypy3},
+    {py36,py37,py38,py39,pypy3},
     report
 
 [testenv]
@@ -14,6 +14,7 @@ basepython =
     py36: {env:TOXPYTHON:python3.6}
     py37: {env:TOXPYTHON:python3.7}
     py38: {env:TOXPYTHON:python3.8}
+    py39: {env:TOXPYTHON:python3.9}
     {doc,spell,bootstrap,clean,check,report,codecov}: {env:TOXTOOLPYTHON:python3.8}
 setenv =
     PYTHONPATH={toxinidir}/tests