package/python-pymupdf: depend on python3

According to the package documentation, python 3.6 or newer
is needed.

Fixes:
 - http://autobuild.buildroot.net/results/f857986b8c9b6d4a843f3694cae8527760540ed0/



Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>