gh-139398: Add supported sunder names to Enum __dir__ for REPL completions

[RafaelWO]

This change adds the sunder names _generate_next_value_ and _missing_ to the __dir__ method of EnumType and Enum.
In Addition, The instance level sunder names _add_alias_ and _add_value_alias_ are added to Enum.__dir__.

With the sunder names exposed in the dir() method, the REPL autocomplete will also show them.

Note: This PR is based on the first attempt to address this issue: #139418

Preview:

>>> from enum import Enum
>>> Enum._
Enum._add_member_(                  Enum._create_(               Enum._find_new_(         --> Enum._missing_(
Enum._check_for_existing_members_(  Enum._find_data_repr_(   --> Enum._generate_next_value_(  
Enum._convert_(                     Enum._find_data_type_(       Enum._get_mixins_(           
>>> class Foo(Enum):
...     BAR = 1
... 
>>> Foo.BAR._   # all of the below are "new"
Foo.BAR._add_alias_(            Foo.BAR._add_value_alias_(      Foo.BAR._generate_next_value_(  Foo.BAR._missing_(

• Issue: REPL does not show completions for some members of the Enum module #139398

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📚 Documentation preview 📚: https://cpython-previews--139985.org.readthedocs.build/

[RafaelWO]

I tried to adjust this test to expect the new attributes, but I wasn't able to get the test to pass. I guess I need some help here 🙂 thanks!

./python -m unittest -v test.test_enum.TestStdLib.test_inspect_classify_class_attrs

[StanFromIreland]

Please don't force push, it complicates reviewing. All commits are squashed at the end anyway.