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

[Daniel Holth]

There is also the 'generator' field in pydist.json : "generator":
"bdist_wheel (0.22.0)". At the time I decided to do pydist.json I
thought I was tracking a spec that would be done any day now, but it
didn't work out that way. Do I recall an earlier complaint that I got
the filename wrong?

It's a bit silly that tools require draft-compliant pydist.json currently.

Calling it version 3 is a fine solution, and the spec has changed
enough from key/value 2.0 to json 2.0, and grown a year older, that
it's also appropriate to bump the version again. A follow on to the
unfinalized 1.3 started in 2005.