Deprecate `_sdl2.window` module

[yunline]

_sdl2.window is added in #2114 for backward compatibility.
Since the old Window cython sources have been removed, I think it's time to deprecate _sdl2.window now.

• Raise a warning when _sdl2.window is imported
• Remove _sdl2.window

[Starbuck5]

I think it's time to deprecate _sdl2.window now.

I disagree. The replacement is not "done"-- it's not documented or publicly exposed or finalized. Deprecation of _sdl2(.video).window can happen after pygame.Window or pygame.display.Window is publicly released.

Even once a replacement is fully released, I think we should keep the alias for a long time, as it doesn't hurt us to do so, and helps immensely with backwards compatibility.