Support wagtail 4.2

[nickmoreton]

This updates the package to support wagtail 4.2

Adds:

• Tox and CI tests for wagtail > 4.1

Removes:

• Support for wagtail < 4.1

[katdom13]

Hi @nickmoreton ,
The changes look good. Thanks.

[gasman]

Thanks @nickmoreton! Just to confirm - given that this is just bumping versions in CI and removing fallback code for older versions, I take it there are no compatibility issues with the current release against Wagtail 4.2 that would necessitate making a new release now, and so we're fine to just keep these updates on the git main branch for now?

[nickmoreton]

Thanks @gasman If we see any issues will report back. 👍