bpo-44342: [Enum] changed pickling from by-value to by-name (GH-26658) (GH-26660)

by-value lookups could fail on complex enums, necessitating a check for
__reduce__ and possibly sabotaging the final enum;

by-name lookups should never fail, and sabotaging is no longer necessary
for class-based enum creation.
(cherry picked from commit 62f1d2b3)

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

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