It appears msgpack-python is Apache 2: https://github.com/msgpack/msgpack-python/blob/master/COPYING.

It does use Cython, but has a built-in pure-python fallback: https://github.com/msgpack/msgpack-python/blob/master/msgpack/fallback.py and https://github.com/msgpack/msgpack-python/blob/master/msgpack/__init__.py#L21-L28