PEP 705: Simplify and clarify proposal

[alicederyn]

Address feedback from @erictraut, plus fix issues I spotted in the process:

• readonly=True appears to be more confusing than expected, and is not strictly necessary, so remove it (see Rejected Alternatives for more)
• The landscape has changed around type discrimination, making it less important to address as part of this PEP, so this is fully deferred to PEP-728 (again, see Rejected Alternatives for more)
• The Python docs use "item" not "entry", so standardize on that term
• There is no explicit name given in the Python docs for d1 | d2, but "merge" is clearer than "union", which is used for sets
• The merge section incorrectly required consistency between A and C, even if the value in A could never end up in C due to it being required in B
• The merge section incorrectly allowed keys to be missed off A and B in a way that could allow unsound typing
• As other_keys is no longer an option for safe updates to TypedDicts with read-only entries, highlight that updates are safe if the bottom type is used to explicitly exclude a key, and require type checker support
• copy and deepcopy were mentioned as having similar behaviour to merge, but it was not clear if this was intended to change how type checkers behave; add a new section explicitly laying out how copy and deepcopy should work for TypedDicts, so this can be discussed
• 2023-10-23 The "union operation" section has been removed completely after discussion on this PR (see Rejected Alternatives)

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📚 Documentation preview 📚: https://pep-previews--3504.org.readthedocs.build/

[erictraut]

Current changes look good to me — at least sufficient for getting another round of feedback from the broader typing community.

[alicederyn]

Re-requesting review as I have made significant changes: the sections on merge and copy/deepcopy have been removed, with explanation on Rejected Alternatives.

[erictraut]

I added a couple of minor comments, but I think this draft is looking really good!

[alicederyn]

Admin question: when this merges, should I continue to use the existing discussion thread on typing-sig, and just post an update? Or should I open a new one?

[JelleZijlstra]

Admin question: when this merges, should I continue to use the existing discussion thread on typing-sig, and just post an update? Or should I open a new one?

I think you should continue the existing thread. Not sure we have firm guidelines, but I would only start a new thread if the proposal becomes radically different or the thread has become very long.