[ENH] Allow customization of cpdef enum to Py conversions

[da-woods]

From a user's perspective, this caused surprise for a project where we weren't using bitwise features of enums and had Python code making decisions based on whether values were iterable. After this change, our enumerations suddenly became iterable and things broke (vapoursynth/vapoursynth#1046).

It's understandable why the switch to IntFlag was made and we've worked around it but I would love if you could optionally signal plain IntEnum behavior outside of C++ context.

Originally posted by @JustinTArthur in #4877 (comment)

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DW's notes:

• It probably isn't too a hard to implement from a code-gen point of view it's just a case of finding a nice syntax. Probably just a decorator on cpdef enum?

• We're obviously slightly at the mercy of what CPython decides to do with their enums. IntFlag wasn't actually iterable until 3.11 (which is admittedly after we started using it) so this change to behaviour is only partly of our own doing.

• I'm not hugely convinced it's worth the extra complication right now.