Add version information macros.

[dlon]

Adds the following macros:

• PG_MAJOR_VERSION
• PG_MINOR_VERSION
• PG_PATCH_VERSION
• PG_VERSIONNUM(MAJOR, MINOR, PATCH) (1000*(MAJOR) + 100*(MINOR) + (PATCH))
• PG_VERSION_ATLEAST(MAJOR, MINOR, PATCH)

Analogous macros exist in SDL.

setup.py makes sure the version info is consistent between the version module, setup script, and header.

[dlon]

If pygame 2.0 shouldn't contain the new features, we can use

#if PG_VERSION_ATLEAST(2, 1, 0)

[illume]

Very nice.

[illume]

Great. Your new version macro will come in handy if we have some experimental features we don't want people to rely on.

By the way... Not sure if I told you or just wrote in the Discord...

I started labeling the issues with milestone 2.1 for things which are new SDL2 additions, and left he 2 milestone for things that are compatibility issues. This way it's a bit easier for us to see when pygame 2 is ready. However, I think it's fine for us to include SDL2 features in pygame 2 if they are ready.