bpo-32513: Make it easier to override dunders in dataclasses. (GH-5366)

bpo-32513: Make it easier to override dunders in dataclasses. (GH-5366)

Class authors no longer need to specify repr=False if they want to provide a custom __repr__ for dataclasses. The same thing applies for the other dunder methods that the dataclass decorator adds. If dataclass finds that a dunder methods is defined in the class, it will not overwrite it.