[docs] [issue17352] Be clear that __prepare__ must be declared as a class method

[Nick Coghlan]

Nick Coghlan added the comment:

__new__ is a little weird - it's actually special cased as a staticmethod.

Your questions is still valid, though.

For existing versions, documenting the requirement is the only option. 

For future versions, we could conceivably implement a "decorate it if it isn't already decorated" fallback, but for backwards compatibility we'd have to avoid double-decorating explicitly decorated implementations.

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