py3: Add support for python3.9

Enable python3.9 in tox.ini and zuul gate. Tested locally
by running tox and running in the zuul gate.

Story: 2009101
Task: 43105

Signed-off-by: Charles Short <charles.short@windriver.com>
Change-Id: I44eaeb134d6b7b54788469fa469b04674aeb90c0

cgcs-patch/cgcs-patch/tox.ini

@@ -5,7 +5,7 @@
 #
 
 [tox]
-envlist = flake8,py27,py36,pylint,cover
+envlist = flake8,py27,py36,py39,pylint,cover
 minversion = 2.3.2
 skipsdist = True
 
@@ -57,6 +57,18 @@ commands = {[testenv:stestr]commands}
 basepython = python3.6
 commands = {[testenv:stestr]commands}
 
+[testenv:py39]
+basepython = python3.9
+install_command = pip install \
+    -v -v -v \
+    -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/starlingx/root/raw/branch/master/build-tools/requirements/debian/upper-constraints.txt} \
+    {opts} {packages}
+commands =
+  find . -name "*.pyc" -delete
+  stestr run {posargs}
+  stestr slowest
+
+
 [bandit]
 # B101: Test for use of assert
 # B104: Test for binding to all interfaces