[Distutils] The PEP 426 defined metadata version will be metadata 3.0

[Daniel Holth]

Unfortunately "only follow the PEP-ratified standards" means "stop
doing useful packaging" until we have some additional standards.

Vinay had mentioned the pypy ABI tagging. I'd hoped implementations
would be interested in figuring this out, but I'm not sure the PyPy
group is or should be very excited about even having an ABI or a
preferred one from the many that their compilation process surely can
generate... for them, the best case might even be to call a library
with cffi, using no Python ABI at all. (the py2.py3-none-x86_64 style
of PEP 425 tag). So I still don't know the right answer for that part
of PEP 425.