Convert `mouse` APIs to output floats

[Starbuck5]

Currently things like mouse.get_pos, mouse.get_rel, pos and rel attributes of MOUSEBUTTONUP and MOUSEBUTTONDOWN, x and y attributes of MOUSEWHEEL are all output as int values.

See #2428 and #2429.

In SDL3, the underlying SDL APIs output floats, so we should as well. In case this causes someone's code to break, I say we push this for a pygame3/sdl3 release. We don't want to make a bunch of huge changes in a pygame3/sdl3 release, but in my opinion this is a good change to make then.

[MyreMylar]

Add a deprecation warning beforehand to let users know that the output of these functions will be changing to floats in 3.0,0?

I can't envisage that many scenarios where this change requires significant user code changes.

[Starbuck5]

I don’t see how a deprecation warning would work, given that it’s the same function that will return different types on different versions.

Really I think most people won’t notice, I’m just saying to leave it for pygame-ce 3.0 to be safe.