Fixed a sentence in dataclasses.rst
Changed "__setattr__" to "object.__setattr__" in a section that was specifically supposed to refer to the __setattr__ method of the object class. Also suppressed the link to the data model docs for __setattr__, since we're talking about a specific __setattr__ implementation, not __setattr__ methods in general.
There is a tiny performance penalty when using ``frozen=True``:
:meth:`~object.__init__` cannot use simple assignment to initialize fields, and
-must use :meth:`~object.__setattr__`.
+must use :meth:`!object.__setattr__`.
Inheritance
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