Get tox working (locally and in CI)

[jams2]

See:

• cookiecutter-wagtail-package's test action
• cookiecutter-wagtail-package's tox.ini
• Dan's comments on https://github.com/torchbox/tbxforms/issues/24

It looks like pytest is the test runner used on this this project so we'll have to make sure tox calls pytest.

---

• [ ] Tox runs the tests for Pull Requests
• [ ] Tox runs the tests locally
• [ ] pytest is used as test runner
• [ ] pytest can be run locally on its own

[jams2]

Fixed in https://github.com/torchbox/tbxforms/commit/ac83e998c49c5753583904927fce8f2a66886012 . Thanks @rachelhsmith !