Use TypeVars rather than specific unions

[max-sixty]

Also from the chars of #8208 -- this uses the TypeVars we define, which hopefully sets a standard and removes ambiguity for how to type functions. It also allows subclasses.

Where possible, it uses T_Xarray, otherwise T_DataArrayOrSet where it's not possible for mypy to narrow the concrete type down.

[max-sixty]

The failure is coming from the docs — getting lots of timeouts; I think unrelated to the code changes

[headtr1ck]

I'm not really a huge fan of using TypeVars if you don't have a return type that uses the same TypeVar.
For this you could also simply define a TypeAlias.

But you are right, at least it unifies things.

[max-sixty]

Yeah, fair.

Your call on whether it's an improvement.

I think if we just have DataArray | Dataset around the place, that's totally fine. The thing I was less keen on is a bunch of different ones that then mean we're digging through mypy errors to understand the cause of something. But actually that hasn't been an issue so far. So no objections to closing...

[max-sixty]

Closing — thanks for the review & useful feedback @headtr1ck , greatly appreciated!