Allow installation on Windows

[emmatyping]

Note: this does not allow one to use the full Python for Android toolchain. It simply removes the requirement for the sh module on Windows so that it is pip installable.

This addresses #819. I am not sure if a warning about the limited abilities is required, but this does allow one to pip install or use setup.py install. When I run p4a (for the curious), I get:

C:\Users\ethanhs>p4a
ERROR: The sh Python module could not be found, please install version 1.10 or higher
python-for-android is exiting due to the errors above.

Something that I could modify for this PR is to change check_python_dependencies to not error on Windows, and instead add a message to wait for binary redistributable recipes.

[inclement]

Thanks for the PR, I guess it's harmless to include it even though p4a doesn't run.