add LICENSE to source distribution

[bollwyvl]

Thanks for this package!

This ensures the LICENSE file is packaged along with the source. While not strictly required for compliance with the MIT license, a number of downstream package management systems strongly encourage self-documenting packages.

Motivation: packaging this for conda-forge. At present, I'm also using the source from github to run the unit tests, and having to patch such that it can test the as-installed package, but that's much more subjective.