python-markdown: new package

[Thomas:
 - Remove host variant of python-markdown, as it is not needed.
 - Add runtime dependency on Python XML module.]

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>