MAINT: split bundled licenses into a separate file

[grlee77]

The motivation here is to help GitHub correctly auto-identify the LICENSE as MIT.

(following the example of SciPy who did this recently for the same reason)

[grlee77]

no point in checking CI here. After merging the license is now listed as "MIT" again, so it worked.

[rgommers]

the thing to check is that this does get included in the wheel (with bdist_wheel)

[grlee77]

Thanks, I checked and it seems to get included (probably via include *.txt in MANIFEST.in)

[grlee77]

It looks like I neglected to include a copy of the NumPy and SciPy license files as part of this PR. I can make a follow-up to add a copy under /doc or something.

[rgommers]

Hmm yeah, you're right that that's technically required I think. No one does this though, community projects in the ecosystem liberally share code; stuff regularly moves between NumPy, SciPy, Statsmodels, Scikit-learn, Scikit-image, etc. Having everyone include all of each others' licenses would become very messy. So happy to leave as is.