✅ Fix broken sdl2 mixer test

[AndreMiras]

Reproduce with:

PYTHONPATH=. pytest tests/recipes/test_sdl2_mixer.py

The error was:

=================================== FAILURES ===================================
__________________ TestSDL2MixerRecipe.test_get_include_dirs ___________________
self = <test_sdl2_mixer.TestSDL2MixerRecipe testMethod=test_get_include_dirs>
    def test_get_include_dirs(self):
>       list_of_includes = self.recipe.get_include_dirs(self.arch)
tests/recipes/test_sdl2_mixer.py:12:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
pythonforandroid/recipes/sdl2_mixer/__init__.py:13: in get_include_dirs
    os.path.join(self.ctx.bootstrap.build_dir, "jni", "SDL2_mixer", "include")
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
a = None, p = ('jni', 'SDL2_mixer', 'include')
>   ???
E   TypeError: expected str, bytes or os.PathLike object, not NoneType
<frozen posixpath>:76: TypeError

Also do some minor code clean up and absolute import path fixes.