Thinking about this some more I am partially reversing course.  The idea behind `IntEnum` and `IntFlag` is to be, as close as possible, drop-in replacements for existing integer-based constants.  If the format() of those two change then they become much less attractive.

So it won't.  Instead, any user-mixed enumerations will get the new behavior:

    class Grades(int, Enum):
        A = 5
        F = 0

will emit a DeprecationWarning now, and in 3.12 `format(Grades.A)` will product 'A' instead of '5'.