dev-python/ruamel-yaml-clib: new package, sort of

dev-python/ruamel-yaml-clib: new package, sort of

This used to be part of dev-python/ruamel-yaml prior to its version
0.16 (which is why why have already got ~arm64 and ~x86 in KEYWORDS),
now however it is a (theoretically optional) separate package.

Assigning myself as maintainer for the time being, that said I'll be
happy to transfer it to Andrey and Foster once they have reported back
on the subject of ruamel.yaml version bump.

Bug: https://bugs.gentoo.org/711070
Signed-off-by: Marek Szuba <marecki@gentoo.org>