Add positional-only parameters

With PEP 570, positional-only parameters were introduced.
Similarly to keyword-only parameters, I would like to see these as an option for attrs.
One thing that makes them "special" is that the __repr__ method would behave differently for positional-only parameters than it would for regular positional parameters.

An example I would use in my libraries:

from attr import attrib, attrs


@attrs(auto_attribs=True)
class Point:
    """A three-dimensional point."""
    _data: Tuple[float, float, float] = attrib(positional_only=True)


print(Point((0, 1, 2)))

The output would then be

Point((0, 1, 2))

instead of

Point(_data=(0, 1, 2))

If this were a desirable feature, would this be a good "first pull request" I could follow up on?

Start with PEP 570 and the issue's Point example, then examine how attrs' attrib option and generated repr currently handle parameters. Define the expected positional-only behavior and verify that the example produces Point((0, 1, 2)) rather than Point(_data=(0, 1, 2)).