python-mistune: new package

Signed-off-by: Mauro Condarelli <mc5686@mclink.it>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>

[Thomas:
 - rename the .mk file to carry the proper name
 - rewrap Config.in help text to use the proper length and avoid a
   trailing space
 - remove excessive empty new line at the end of Config.in
 - specify BSD-3c as the license, since it's the variant being used,
   and it's more specific than just BSD.]

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