package/python-lxml: new package

The lxml XML toolkit is a Pythonic binding for the C libraries libxml2
and libxslt.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
[yann.morin.1998@free.fr: fix staging path; trim dependencies of
 the host variant]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>