gh-112328: Make EnumDict usable on its own and document it

[encukou]

This is a continuation of @u2rafi's PR #121720, with @AA-Turner's review comments addressed.

@ethanfurman, I noticed EnumDict relies on a private attribute, so I made that attribute optional, and added tests.
Is this what you intended when making EnumDict public? Or was it more for typing and isinstance rather than using it in custom __prepare__?

• Issue: [Enum] make some private attributes public #112328

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📚 Documentation preview 📚: https://cpython-previews--123669.org.readthedocs.build/

[encukou]

@ethanfurman, could you clarify how EnumDict should be used?

[encukou]

@ethanfurman, could you clarify how EnumDict is meant to be used?

[miss-islington-app]

Thanks @encukou for the PR, and @ethanfurman for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @encukou and @ethanfurman, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 2a66dd33dfc0b845042da9bb54aaa4e890733f54 3.13

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot aarch64 RHEL8 LTO 3.x has failed when building commit 2a66dd3.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/338/builds/7721) and take a look at the build logs.
4. Check if the failure is related to this commit (2a66dd3) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/338/builds/7721

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/threading.py", line 1054, in _bootstrap_inner
    self.run()
    ~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/threading.py", line 996, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/test/test_interpreters/test_stress.py", line 30, in task
    interp = interpreters.create()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/test/support/interpreters/__init__.py", line 76, in create
    id = _interpreters.create(reqrefs=True)
interpreters.InterpreterError: interpreter creation failed
k


Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/threading.py", line 1054, in _bootstrap_inner
    self.run()
    ~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/threading.py", line 996, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/test/test_interpreters/test_stress.py", line 47, in run
    interp = interpreters.create()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/test/support/interpreters/__init__.py", line 76, in create
    id = _interpreters.create(reqrefs=True)
interpreters.InterpreterError: interpreter creation failed
k

[bedevere-app]

GH-128142 is a backport of this pull request to the 3.13 branch.