Start compiling _sdl2 with SDL3

[damusss]

I have started working on compiling pygame._sdl2 with SDL3. This PR is draft because I'd like to port touch and audio, I'm creating it early to get feedback and opinions, especially for the following submodules:

• We don't have to port video, right? I mean, it's being ported as pygame.render which hopefully is already sdl3 compatible. No reason porting obsolete code to SDL3
• Same thing for window. pygame.window is already ported, there's not reason to port this too, since it's also outdated.
  Do you agree?
  (also I want to see what the checks have to say about my changes)