Add support to test with Django 3.0

[cwdavies]

Add support to test with Django 3.0

Additions

• dj30: Django>=3.0,<3.1 in tox.ini

Changes

• envlist=lint,py{36,38}-dj{22,30} in tox.ini
• Django>=2.2,<3.1 in setup.py

Testing

• tox -e py36-dj30
• tox -e py38-dj30

Review

• @user

Preview this PR without the whitespace changes

Checklist

• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [ ] Code follows the standards laid out in the front end playbook
• [x] Passes all existing automated tests
• [ ] New functions include new tests
• [ ] New functions are documented (with a description, list of inputs, and expected output)
• [ ] Placeholder code is flagged
• [ ] Visually tested in supported browsers and devices
• [ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)