devise a process for keeping packages up to date

[chris48s]

There is a need to keep this package up-to-date as new versions of python/django/wagtail are released. This does require some manual intervention to update the trove classifiers, tox matrix, etc. However it would be nice to have a bot submit a PR as a prompt for me to do this when a new release is available.
Unfortunately, for this repo it is not as simple as just pointing dependabot at it and telling it to bump the ranges because our pyproject.toml is (will be) a valid jinja template, but it is not valid toml.
Need to have a think about how to approach this.