package/python-pycups: needs python3

As stated in README:

Python 3 or later is required since Python 2 is unsupported since
2020-01-01.

Fixes:
 - http://autobuild.buildroot.org/results/3c8db83a2d1ae094fc718947a508e35e940121ce



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>