Port intervals from msprime, changing licence

The authors of this module and associated tests have agreed to release the code originally developed for the msprime package under the MIT licence (see tskit-dev#2636). Specifically:

@jeromekelleher: "I approve the relicensing of this code originally developed in msprime from GPLv3 to MIT."

@hyanwong: "I am happy to relicense all of my contributions to this code (originally developed in msprime) from GPLv3 to MIT."

@awohns: "I also approve the relicensing of this code originally developed in msprime from GPLv3 to MIT."

@grahamgower: "I approve the relicensing of this code originally developed in msprime from GPLv3 to MIT."

@petrelharp: "I approve the license change!"

Author: hyanwong