Raise an AttributeError for __getattr__ methods in AthConfigFlags

Jon Burr requested to merge jburr/athena:ATEAM-898_23.0 into 23.0

Closes ATEAM-898.

@amete noted that the existing approach of raising RuntimeError (FlagAddress) or just blindly creating a FlagAddress object (AthConfigFlags) breaks the use of getattr. It also violates basic python rules...

This unifies the logic between the two implementations and ensures that an AttributeError is raised as required by python.

This replaces !62173 (closed) now targeting 23.0.

Edited by Jon Burr

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