⬆️ Quality of Life Updates

[jefftriplett]

This pull request:

• [x] Add Python 3.9 support
• [x] Supports Python 3.6 and newer
• [x] Support Django LTS 2.2 and 3.2
• [x] Migrates url() to path() calls which should help with some previous reverse lookup issues
• [x] Updates Trover Classifiers and drops the unused VERSION import
• [x] Updates requirements.* files to use pip-tools
• [x] Upgrades/lints setup.* format
• [x] :fire: Drops Travis support
• [x] :fire: Drops Tox support
• [x] :fire: Drops Django <2.0 (possibly drops Django<2.2 as well)
• [x] :fire: Drops Python <3.6 support (in theory it might still work)