Add Python 3.9 support

This has been out since October. Time to support it.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

releasenotes/notes/python-3-9-support-79fe9f9cbb1bf2b0.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    `Python 3.9 <https://www.python.org/downloads/release/python-390/>`_ is now
+    supported.

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.2
-envlist = pep8,docs,py{36,37,38}-django{22,30,31}
+envlist = pep8,docs,py{36,37,38,39}-django{22,30,31}
 skipsdist = true
 ignore_basepython_conflict = true
 
@@ -22,7 +22,7 @@ setenv =
     PYTHONDONTWRITEBYTECODE = 1
     PYTHONDEVMODE = 1
     py36: PYTHONWARNINGS = once,ignore::ImportWarning:backports
-    py{37,38}: PYTHONWARNINGS = once
+    py{37,38,39}: PYTHONWARNINGS = once
 passenv =
     http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
     PW_TEST_DB_TYPE PW_TEST_DB_USER PW_TEST_DB_PASS PW_TEST_DB_HOST