tox.ini: Add Python 3.9 to the testing
diff --git a/tox.ini b/tox.ini
index d74138a..b87e5f1 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 envlist =
-    py27,py34,py35,py36,py37,py38,pypy,pypy3,flake8
+    py27,py34,py35,py36,py37,py38,py39,pypy,pypy3,flake8
 
 [testenv]
 deps =