This is required by RFC 1542, so despite the subtle behavior change we
are treating it as a bug.  Patch by Michael Lazar.

pretty early on in the 3.x series (3.1 or 3.2?).

Builds on non-OSX UNIX now default to using the system libffi, and warn if the
bundled copy is used.

Thanks to Georg Brandl for the patch.

now is interned. This leads to more compact pickle data with protocol 4.