Create an extension for dataclass based classes

[tristanlatr]

@dataclasses.dataclass can be used from python3.7, it acts a bit like @attr.s, so adding support for it shouldn't be too complicated.

Also I realize that there are many modules that converts class variables into instance variables (pydantic, dataclasses, attrs, django models, and others), so there is probably something to do to avoid to much code duplications.

[tristanlatr]

Same goes for namedtuples.

[tristanlatr]

And eventually the datatclass decorator?