package/python-yieldfrom: fix licensing info

There is no file containing the license text. The closest we can use is
the PKG-INFO file, which contains the license name...

Fixes:
    http://autobuild.buildroot.net/results/dff21d63845d3d6a5f78c1000301cdd2104595f4
    http://autobuild.buildroot.net/results/220e82f555ce6c98b89605bab72c7ea3eea61c66


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Asaf Kahlon <asafka7@gmail.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>