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

[Nick Coghlan]

Vinay brought to my attention that bdist_wheel currently publishes
pydist.json files that claim to contain content in the metadata 2.0
format. I was aware that Daniel had been experimenting with generating
pydist.json files, but I didn't realise that bdist_wheel did it by
default, nor that it used the real file name from the PEP rather than
a separate implementation specific file.

That's broken: it means that metadata format 2.0 can now never be well
defined, because we already have these experimental (and ultimately
non-conformant, since there have been backwards incompatible changes
to the draft spec) files floating around.

I will accordingly be updating the defined metadata version in PEP 426
to 3.0, and including an explicit admonition to *never* include
experimental metadata (whether in the base format or as part of an
experimental extension) in the main pydist.json file. Experimental
metadata should only ever appear in tool-specific files (which don't
need to guarantee any kind of interoperability with other tools).

In the meantime: please don't publish pydist.json files, use some
other filename if you want to experiment with JSON based metadata in
advance of the acceptance of PEP 426.

Regards,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia