PEP 788: Address feedback from first discussion round

[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)

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The key changes I made here are:

• Interpreter references now have a dedicated type (PyInterpreterRef and PyInterpreterWeakRef *).
• Add a terminology section for clarification on what things like "interpreter" refer to in the PEP.
• Add a rejected idea for the previous approach (mainly using PyInterpreterState * for refs).
• Shifted away from "non-daemon thread states," so now interpreter references and thread states are
  kept separate.
• Editorial: Lots of clarity fixes (many had trouble understanding the PEP, unfortunately).
• Editorial: Headings are now in title case ("The Apple is Weird and Confusing") rather than sentence case ("The apple is weird and confusing").

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📚 Documentation preview 📚: https://pep-previews--4400.org.readthedocs.build/pep-0788/

[ZeroIntensity]

@godlygeek I can't add you as a reviewer, but I'd appreciate your opinion on some of the new things I'm proposing here.

[ZeroIntensity]

Bump @AA-Turner @vstinner now that the beta is out. There's no rush considering we have a year until the 3.15 freeze, though.

[ericsnowcurrently]

Thanks for working on this. I haven't gotten though the whole proposal, but did start by trying to understand what we're trying to solve here. I've left short notes for each of the motivation sections. Ultimately, I think are some bugs to be addressed separately and the other concerns could be solved without replacing the existing API. That doesn't mean a new API isn't warranted, but let's see how much value it would add first. (I'd be glad to hop into a call to discuss this more.)

[ZeroIntensity]

I'd be glad to hop into a call to discuss this more.

That'd be great. Shoot me a message on Discord so we can figure something out. (Note that I'll have limited availability this week.)

[ZeroIntensity]

I think most of Eric's comments will fit better on the public discussion thread, so I didn't deal with them too much.

@colesbury I've addressed most of your concerns. In particular, thread states and interpreter refs are now separate, so there's no more "non-daemon thread states." I'd appreciate your rubber-stamp before landing this, would you mind doing a quick once-over when you get a chance?

[ZeroIntensity]

I had to resolve many of the comments because it won't let us merge this while things are open. If there's anything that wasn't fully addressed, please bring it up on the thread!

@vstinner, we're ready to go.

[vstinner]

I merged the PR, thanks.