gh-103596: [Enum] do not shadow mixed-in methods/attributes

[ethanfurman]

For example:

class Book(StrEnum): title = auto() author = auto() desc = auto() Book.author.desc is Book.desc 

but

Book.author.title() == 'Author' 

Using upper-case member names avoids any possible confusion.

• Issue: [Enum] attributes of mixed-in classes can be shadowed by other members #103596

[ethanfurman]

Note that performance is only impacted when there is a name clash between member and existing attribute/method.