PEP 566 defines a metadata 2.1 specification that
matches the way tools are actually currently working,
so we're going to run with that "solve problems we
know we have", rather than pursuing the more
speculative ground-up metadata redesign that PEP 426
became.
This is part of a larger refactoring of PEP 426, but I needed
to commit this early in order to claim the PEP number and
cross reference it from PEP 426 and 440.