bpo-43945: [Enum] reduce scope of new format() behavior (GH-26752)

* [Enum] reduce scope of new format behavior

Instead of treating all Enums the same for format(), only user mixed-in
enums will be affected.  In other words, IntEnum and IntFlag will not be
changing the format() behavior, due to the requirement that they be
drop-in replacements of existing integer constants.

If a user creates their own integer-based enum, then the new behavior
will apply:

    class Grades(int, Enum):
        A = 5
        B = 4
        C = 3
        D = 2
        F = 0

Now:  format(Grades.B)  -> DeprecationWarning and '4'
3.12:                   -> no warning, and 'B'