Utility repository for building multi-platform wheels for UDon2

The building is done through Travis CI for Linux and macOS and AppVeyor for Windows using the multibuild utilities.

Currently supported platforms:

• Linux (i686 and x86_64) for Python 3.6 - 3.9
• Windows (win32 and win_amd64) for Python 3.7 - 3.9
• macOS (x86_64) for Python 3.7 - 3.9