Django-request-token

No functional changes - just updating classifiers and CI support.

Added

- Add Django 3.1 support
- Add Django master to CI build matrix

Added

- Add `isort` and `black` formatting
- Add `pylint` and `flake8` linting
- Add type hints and `mypy` checks
- Add pre-commit config

Changed

- Replace pipenv with poetry

Deprecated

- Django 1.11, 2.0, 2.1
- Python 3.6