Add Wagtail 6 to testing matrix

[mgax]

The default testing matrix only includes Wagtail 5.2. I wonder if this should be testing against dev versions of Wagtail too? It would help surface compatibility issues sooner.

Here's a successful test run. It's based off the #76 branch.

[chris48s]

Thanks.
The cookiecutter does already give you a nightly build that runs tests against whatever is on main in the wagtail repo
https://github.com/wagtail/cookiecutter-wagtail-package/blob/main/%7B%7B%20cookiecutter.__project_name_kebab%20%7D%7D/.github/workflows/nightly.yml
This helps surface possible compatibility problems, but it means it isn't a requirement for pushes/PRs.

[chris48s]

While we're doing this, could you also add the wagtail 6 framework classifier in pyproject.toml. Aside from that, looks good.

[mgax]

While we're doing this, could you also add the wagtail 6 framework classifier in pyproject.toml. Aside from that, looks good.

Ah, good catch, thanks! I've added the classifier.