PEP 747: Explicitly define "valid type expression".

[rchen152]

When I was reading the PEP, it was unclear to me whether the rule on what expressions can implicitly evaluate to TypeForm applied to the rest of the PEP. This PR suggests pulling that rule out into its own section and linking to it the first time "valid type expression" is used, to make it clear that this definition applies throughout.

• Change is either:
  • To a Draft PEP
  • To an Accepted or Final PEP, with Steering Council approval
  • To fix an editorial issue (markup, typo, link, header, etc)
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

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📚 Documentation preview 📚: https://pep-previews--4484.org.readthedocs.build/

[hugovk]

Ready to merge?

[rchen152]

cc @davidfstr @erictraut (just realized that I never notified the authors of PEP 747 of this PR, oops!)

[erictraut]

Thanks @rchen152. The proposed change looks good to me.

[davidfstr]

Also looks good to me 👍

(And hi, I'm alive. I'm still trying to find enough cycles to push the last mypy+TypeForm PR through. This upcoming weekend will be the first in 4 weeks when I don't have high priority commitments. Fingers-crossed I can get it moved forward then.)