WIP: Update to Django 5.1

[StevenMaude]

Fixes #2115.

This PR is to:

• preserve work-in-progress commits from Bump django from 4.2.16 to 5.1.4 #2198…
• …which allows us to close off all the Dependabot Python upgrades, now that we're no longer using Dependabot for those upgrades

[StevenMaude]

The issue here is with coding systems test failures, due to the way the tests use databases.

[StevenMaude]

This also needs local testing as previously to ensure everything is OK with the upgrade.

[StevenMaude]

Current status of this:

• the working tests do pass, but are a bit of a mess still, so this PR does look somewhat horrendous right now, until we can get everything working, and then refine it
• the use of pytest fixtures, that those fixtures aren't easily available on classes when you're using unittest.TestCase and, for the base coding system tests, the time at which those fixtures are created, all makes this more complicated
• it's probably possible to move the general modified TestCase class out of each test module into a higher level, in most cases, and remove that duplication
• the import_data_path on the TestCase isn't always needed
• there's a bit of reviewing which pytest fixtures are being used (maybe the settings fixture is needed in places)
• from what I've looked at of the failing base tests, there's some other magic going on with fixtures (bnf_release and maybe others) that still means that the specified db_alias doesn't get picked up by the test, and we get the same DatabaseOperationForbidden error