package/python-webpy: fix licensing information

The license files defined in the .mk do not exist in the release
tarball. However, they do exist in the git repository.

Since the git tree is rather small, switch to using it instead of the
released tarball.

Fixes:
    http://autobuild.buildroot.org/results/d94/d944f36220326510c15685efe19075e90e72a5c3/
    http://autobuild.buildroot.org/results/14f/14f411ef29cbf0ba8cf4a4fed71556eacc96eb04/


    and many others...

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