Optional fields are really optional

[wimglenn]

• make all fields documented as optional in the spec truly optional (and don't spam warnings when they've been omitted)

• stop coercing missing values to the string "UNKNOWN"

Note: the setuptools integration is expected to fail, because this PR needs to be coordinated with changes in setuptools/dist.py too. I'll prepare a separate PR on pypa/setuptools for that part.

[jaraco]

Overall, this looks good to me. Is there any way this change could be rolled out in such a way that it doesn't have to be synchronized with Setuptools? Perhaps to have a PR land there first? I'm concerned the coordination with Setuptools may be indicative of a compatibility issue with stdlib distutils. I'll check the Setuptools PR soon.

[wimglenn]

@jaraco IIUC you want the changes in setuptools to work regardless of whether it calls to pypa/distutils or stdlib distutils?

[jaraco]

@jaraco IIUC you want the changes in setuptools to work regardless of whether it calls to pypa/distutils or stdlib distutils?

That's correct. Setuptools still supports using stdlib distutils, so any change there will need to support both. If this change requires Setuptools to drop support for stdlib distutils, the change will have to wait until that happens (maybe this year).

[wimglenn]

@jaraco It is not necessary to drop support for stdlib distutils. More details here.