  [tox]
  envlist =
         {py35,py36,py37}-django22,
         {py36,py37,py38}-django30,
         {py36,py37,py38}-django31,
         {py36,py37,py38}-djangomaster,
         base,dist,lint,docs,
  
  [travis:env]
  DJANGO =
      2.2: django22
 
</s> remove         django31: Django>=3.1a1,<3.2
 </s> add         django31: Django>=3.1,<3.2
       {py36,py37,py38,py39}-django31,
       {py36,py37,py38,py39}-djangomaster,
  [tox]
  envlist =
         {py35,py36,py37}-django22,
         {py36,py37,py38}-django30,
       {py36,py37,py38,py39}-django31,
       {py36,py37,py38,py39}-djangomaster,
       {py36,py37,py38,py39}-django31,
       {py36,py37,py38,py39}-djangomaster,
         base,dist,lint,docs,
  
  [travis:env]
  DJANGO =
      2.2: django22
keep keep keep keep replace replace keep keep keep keep keep
 Run tests against Python 3.9 (#7517)

3.9.0 final is expected on Monday, 2020-10-05
See https://www.python.org/dev/peps/pep-0596/#schedule

Also mention Django 3.1 compat where missing. 
tox.ini