Tox.ini: add py39

This allows us to run unit tests and functional tests using Python3.9.
We also remove all testenv:py3* environments, since tox is able to infer
what version of Python to use from the environment name.

Change-Id: If9ce1475ddfc66c9b37cd3500af982bf3814bf60

tox.ini

@@ -30,20 +30,12 @@ deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/
        -r{toxinidir}/requirements.txt
 commands =
     find . -type f -name "*.pyc" -delete
+    stestr run --slowest {posargs}
 whitelist_externals = bash
                       find
                       rm
 passenv = *_proxy *_PROXY
 
-[testenv:py36]
-commands = stestr run --slowest {posargs}
-
-[testenv:py37]
-commands = stestr run --slowest {posargs}
-
-[testenv:py38]
-commands = stestr run --slowest {posargs}
-
 [testenv:functional]
 # this will use whatever the system python3 is
 setenv =
@@ -62,6 +54,10 @@ commands = {[testenv:functional]commands}
 setenv = {[testenv:functional]setenv}
 commands = {[testenv:functional]commands}
 
+[testenv:functional-py39]
+setenv = {[testenv:functional]setenv}
+commands = {[testenv:functional]commands}
+
 [testenv:genpolicy]
 basepython = python3
 commands =