set_source_location fails at runtime on some audio setups

[Starbuck5]

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This hasn't been reported on this repo yet, but we really need to do something about it, so making this issue.

When calling set_source_location on certain mixer setups

pygame.error: Unsupported audio channels

Here's the SDL_mixer issue for context: https://github.com/libsdl-org/SDL_mixer/issues/573

This has hit both bigwhoop and dafluffypotato when distributing their games to people where mixer defaults to 7.1 surround on their setup.

• Should we document how to force stereo in the docs for this function and tell people to force stereo?
• Can we prevent mixer from opening in 7.1 mode somehow?
• We could make this a silent fail instead of an exception? Maybe not a good idea.