python-pyxml: new package

It always uses its own copy of pyexpat, but we can at least make it use
buildroot's expat.so.

[Peter: select expat]
Signed-off-by: Clayton Shotwell <clshotwe@rockwellcollins.com>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>