Add missing py39 to tox

tox.ini

@@ -1,10 +1,10 @@
 [tox]
-envlist = {py36,py37,py38}-{sqlite,postgresql},pypy3-{sqlite,postgresql}
+envlist = {py36,py37,py38,py39}-{sqlite,postgresql},pypy3-{sqlite,postgresql}
 
 [testenv]
 commands = {envpython} setup.py test
 deps =
-    {py36,py37,py38}-postgresql: psycopg2 >= 2.5
+    {py36,py37,py38,py39}-postgresql: psycopg2 >= 2.5
     pypy3-postgresql: psycopg2cffi >= 2.5
     py36-sqlite: sqlitebck
 setenv =