python-txtorcon: bump to version 18.1.0

Also add license hash.

Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
[Thomas: bump to 18.1.0, to fix Python 2.x issue, reported by Ricardo.]
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>