Add typing

[jieter]

Writing union types as X | Y is first added in python 3.10, if we keep supporting it until the end of life of python 3.9, that would mean we can only merge this after 2025-10.

Alternatively, we can use the Union[X, Y] syntax which is already supported by python 3.9.

Another issue is Unpack which is available from python 3.11. It is part of typing_extensions, so we can still use it.

Ideally, users of django-tables2 should still be able to use def render(self, record): or def render(self, value): without complaints of the type checker. I think this might be hard to achieve, so we should document that using def render(self, **kwargs: Unpack[CellArguments]) and unpacking kwargs in the body of the method is the way to achieve type safety.

[mschoettle]

With pyupgrade it should be easy to move from Union to | once Python 3.9 support gets dropped. It looks like the pre-commit config needs a small update to target 3.9 instead of 3.8.

[jieter]

The unions could be removed now that python 3.9 support is dropped. Mypy is satisfied now, the unpacking issue remains.