[bugfix] Fix __getattr__ for WikibaseEntity
[bugfix] Fix __getattr__ for WikibaseEntity

Docs: https://docs.python.org/3.7/reference/datamodel.html#object.__getattr__

Bug: T281389
Change-Id: Ic88559a8bf9bbbdc664a0c29d20e99de2bf38724

wikimedia/pywikibot-core,wikimedia/pywikibot-core