Improve handling of non-string values for 'override_tag's 'default_html' argument

[alxbridge]

Description

Fixes #211

Raises a (more) meaningful TypeError if a call is made to override_tag with something other than a string as the default_html arg. This raised error provides some context for the bad call, which was not provided in the traceback when thrown directly by Django >= 4.0.

If run in Django < 4.0, this PR causes a warning to be logged instead, giving developers on those older versions advance warning of the change.

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] I have added an appropriate CHANGELOG entry

[zerolab]

fwiw, I tested the fix the warning suggests on an active project and that fixed the PL issue

[alxbridge]

NB I've pushed an updated version of this PR with an extra commit to allow the tox config to run. However, it looks like there are other PRs to address this same issue in other ways: if preferable I can remove this again, now I can see all checks have passed.

[alxbridge]

@bcdickinson I believe I've addressed the changes you requested - please could you re-review?