Get tox working (locally and in CI)

[Joshua Munn]

See:

• cookiecutter-wagtail-package's test action
• cookiecutter-wagtail-package's tox.ini
• Dan's comments on #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

[Joshua Munn]

Fixed in ac83e99 . Thanks @rachelhsmith !