gh-93250: [Enum] Change IntEnum boundary to KEEP for backwards compatibility (GH-93302) (GH-93304)

In previous versions of Python if an IntEnum member was combined with another integer type value using a bit-wise operation, the resulting value would still be the IntEnum type.  This change restores that behavior.
(cherry picked from commit 70cfe56c)

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>