         PYTHONWARNINGS=once
  deps =
          django22: Django>=2.2,<3.0
          django30: Django>=3.0,<3.1
          django31: Django>=3.1a1,<3.2
          djangomaster: https://github.com/django/django/archive/master.tar.gz
          -rrequirements/requirements-testing.txt
          -rrequirements/requirements-optionals.txt
  
  [testenv:base]
 
</s> remove        {py36,py37,py38}-django31,
       {py36,py37,py38}-djangomaster,
 </s> add        {py36,py37,py38,py39}-django31,
       {py36,py37,py38,py39}-djangomaster,
        django31: Django>=3.1,<3.2
         PYTHONWARNINGS=once
  deps =
          django22: Django>=2.2,<3.0
          django30: Django>=3.0,<3.1
        django31: Django>=3.1,<3.2
          djangomaster: https://github.com/django/django/archive/master.tar.gz
          -rrequirements/requirements-testing.txt
          -rrequirements/requirements-optionals.txt
  
  [testenv:base]
keep keep keep keep 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