Remove JoinManager class

[mthuurne]

The JoinManager class doesn't provide anything that is not already provided by JoinQueryset.as_manager().

My main motivation for removing JoinManager is that it would be too much work to make JoinManager work properly with type annotations, while as_manager() works out of the box thanks to the django-stubs plugin.

This is a backwards incompatible change, as code using JoinManager or JoinManagerMixin would have to be updated. In most cases, I expect that update to be a single-line change; see the unit test changes for an example.

I don't know whether JoinQueryset is still relevant functionality. It generates SQL as a string and contains custom quote code, which always makes me worried about the potential for injection attacks. Perhaps that class could be removed as well?

[mthuurne]

The docs (docs/managers.rst) will need updating as well. But before spending more time on it, I'd like to know whether you agree with removing JoinManager.

[foarsitter]

The JoinManagerMixin and JoinManager are there only for convenience it looks like.

I don't like breaking code so maybe we should just ignore them for mypy and remove them in de future?

[mthuurne]

I don't like breaking code so maybe we should just ignore them for mypy and remove them in de future?

That would be fine with me.

The main thing I want to avoid is a user getting a worse type checking experience because they're not aware that there is a better supported alternative.

I'll close this PR and open a new one to deprecate JoinManager.