python-webpy: depend on Python 2.x only

webpy is still not ported to Python 3, so enable only for Python 2.

Fixes:

 http://autobuild.buildroot.org/results/e6b/e6bdb8e0ab6daa027a01507861ecde47338a11a8/



[Thomas: add autobuilder reference.]

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>