Allow .framework DLLs on macOS

[a-hurst]

The pre-built SDL2 binaries for macOS on the official website are all packaged in .framework format. However, PySDL2 currently only looks for SDL2 DLLs in .dylib format, meaning that if you download the official frameworks and try to point PYSDL2_DLL_PATH towards them, it won't be able to find them.

This patch fixes this, and lays the groundwork for having a separate 'pysdl2-dlls wheel' that contains the official pre-built SDL2 + extras runtime libraries which would solve #78 (I currently have a script/package to create such a wheel for macOS and it seems to work well using this fix!)

[a-hurst]

In order for the official SDL2_Mixer framework to load on macOS, I also had to comment out the Mix_HasMusicDecoder function in sdlmixer.py. Otherwise, I get the following error:

ValueError: could not find function 'Mix_HasMusicDecoder' in <CDLL '/usr/local/lib/python3.7/site-packages/sdl2dll/dll/SDL2_mixer.framework/SDL2_mixer', handle 7faf27d90d80 at 0x112aac048>

Looking into it further, the Mix_HasMusicDecoder function exists in the SDL_mixer.h header but isn't actually implemented anywhere in SDL_Mixer, meaning that the binding process in Python fails. Thus, there's no loss in functionality by removing the function from PySDL2 until it actually gets implemented.