package/x11r7/xkeyboard-config: needs a host python3

package/x11r7/xkeyboard-config: needs a host python3

A host python3 is needed to run map-variants.py since version 2.29 and
https://gitlab.freedesktop.org/xkeyboard-config/xkeyboard-config/-/commit/57c745a9a373b4c9d272d00ff4f16dbcc6d3c48b

The script (with a backported patch) does not have stringent requirements
on python, so we may use a system python3 if present.

Fixes:
 - http://autobuild.buildroot.org/results/7c3cdecc2feed62d7e0bad0e790a8bacaaa34cb7

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>