Log Message:
On NetBSD current with native threads it seems this python module can't
be imported by a non-thread aware python without it core dumping.
Hence restrict accepted python packages to threaded ones if native
threads are supported by the platform.

Bump PKGREVISION for that.

Provide a buildlink3.mk file mostly to help bl3ified packages to pick up
the right python packages with respect to this threading issue.