Changes:
- Replace ugettext_lazy with gettext_lazy for Django 4.x support
- Remove Signal providing_args for Django 4.x support
- Replace url with re_path for Django 4.x support
CI:
- Add tox tests for Django 3.0, 3.1, 3.2, 4.0, 4.1
- Add new Python versions for Travis tests
- Bump Travis Ubuntu version for Python 3.10
- Bump Postgres (9.2 -> 11) for Django 4.0 to work