python-twisted: bump to version 18.7.0

Add license hash.

Add a patch to fit python3.7 syntax ("async" is now a keyword).
The patch was already applied on upstream, but only after the last tag
was created.

Fixes:
http://autobuild.buildroot.org/results/704/7043ccfac86439406480649b3ead4882112d36e2/



Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>