add py39 environment to tox test envs

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = begin,py37,py38,coverage,security,lint,bundle
+envlist = begin,py37,py38,py39,coverage,security,lint,bundle
 
 [testenv]
 # build a wheel and test it
@@ -41,6 +41,11 @@ commands =
 commands =
     pytest --cov-append --cov={envsitepackagesdir}/incorporealcms/ --cov-branch
 
+[testenv:py39]
+# run pytest with coverage
+commands =
+    pytest --cov-append --cov={envsitepackagesdir}/incorporealcms/ --cov-branch
+
 [testenv:coverage]
 # report on coverage runs from above
 skip_install = true