PEP 788: Minor clarity changes and improvements

[ZeroIntensity]

• 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)

I've done the following here:

1. Removed the ten-year deprecation for PyGILState and instead leave it as undetermined.
2. Made many small changes that improve the clarity of the proposal.
3. Added an "Acknowledgements" section.

I plan to submit PEP 788 to the C API working group for feedback after this is merged.

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📚 Documentation preview 📚: https://pep-previews--4474.org.readthedocs.build/

[vstinner]

LGTM