sdist tar.gz for releases needs its own LICENSE file

[pjfanning]

One minor issue in the LICENSE file for the 'sdist' archive. It
mentions website files that are not in that archive. It looks like the
LICENSE is copied from the src archive and these files are in that.
It would be good to get this fixed for future RCs.

[pjfanning]

fyi @jernejfrank

[jernejfrank]

Resolved in #899