tox: enable python 3.5 & 3.9 testing

We still support Python 3.5, so make sure it keeps working.

Change-Id: I150158a656b26de6d733316a68a2cbb8b5b99716
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/299625
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
diff --git a/tox.ini b/tox.ini
index b4758d1..3282de1 100644
--- a/tox.ini
+++ b/tox.ini
@@ -15,13 +15,15 @@
 # https://tox.readthedocs.io/
 
 [tox]
-envlist = py36, py37, py38
+envlist = py35, py36, py37, py38, py39
 
 [gh-actions]
 python =
+    3.5: py35
     3.6: py36
     3.7: py37
     3.8: py38
+    3.9: py39
 
 [testenv]
 deps = pytest