gh-93464: [Enum] fix auto() failure during multiple assignment (GH-99148)

* fix auto() failure during multiple assignment

i.e. `ONE = auto(), 'text'` will now have `ONE' with the value of `(1,
'text')`.  Before it would have been `(<an auto instance>, 'text')`
(cherry picked from commit 8feb7ab77c80968a6de6079299a39b0494b1701b)

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