Fix RTD Python version
Fix RTD Python version

* Python 3.9 is technically supported via the testing image.
ryankanno/cookiecutter-django,ryankanno/cookiecutter-django,trungdong/cookiecutter-django,pydanny/cookiecutter-django,pydanny/cookiecutter-django,pydanny/cookiecutter-django,ryankanno/cookiecutter-django,ryankanno/cookiecutter-django,trungdong/cookiecutter-django,trungdong/cookiecutter-django,pydanny/cookiecutter-django,trungdong/cookiecutter-django