Add python39 testing

We deploy gerritlib via jeepyb on our gerrit images which have python39
now. Add testing for that version of python to help cover that setup.

We also drop the python3.5 default in tox.ini as we don't test that
version of python anymore.

Change-Id: I4be713a6fe67b6524eb3a463c9835851bc7f90e5

.zuul.yaml

@@ -19,6 +19,8 @@
             nodeset: ubuntu-bionic
         - tox-py38:
             nodeset: ubuntu-focal
+        - tox-py39:
+            nodeset: ubuntu-focal
     gate:
       jobs:
         - gerritlib-jeepyb-integration
@@ -28,3 +30,5 @@
             nodeset: ubuntu-bionic
         - tox-py38:
             nodeset: ubuntu-focal
+        - tox-py39:
+            nodeset: ubuntu-focal

setup.cfg

@@ -20,10 +20,10 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
 
 [files]
 packages = gerritlib

tox.ini

@@ -3,7 +3,7 @@ minversion = 1.6
 skipsdist = True
 skip_missing_interpreters = false
 # https://docs.python.org/devguide/#status-of-python-branches
-envlist = pep8, py{38,37,36,35,27,py}
+envlist = pep8, py{39,38,37,36,27,py}
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}