Support attrs 20.1's on_setattr=attr.setters.convert

Feature

Version 20.1 of attrs introduced support for running a field's converter whenever the field is set. Here's an example of it in action:

import re
import attr

@attr.s(auto_attribs=True)
class GHRepo:
    owner: str
    name: str


def parse_ghrepo(s: str) -> GHRepo:
    if (m := re.fullmatch(r'([^/]+)/([^/]+)', s)) is not None:
        return GHRepo(owner=m[1], name=m[2])
    else:
        raise ValueError(s)


@attr.s
class Foo:
    repo: GHRepo = attr.ib(converter=parse_ghrepo, on_setattr=attr.setters.convert)

f = Foo("owner/name")
f.repo = "python/mypy"  # mypy errors on this line
print(f)

This prints out Foo(repo=GHRepo(owner='python', name='mypy')), as the "python/mypy" assigned to f.repo is converted to a GHRepo with parse_ghrepo().

Unfortunately, mypy as of both version 0.812 and commit 28034cc does not recognize the typing implications of on_setattr=attr.setters.convert, as running it on the above code with the default settings gives the error Incompatible types in assignment (expression has type "str", variable has type "GHRepo") on the marked line. I would like to request that mypy recognize the setting's typing implications and allow thus-configured fields to be set to the input types of their converters.

Pitch

This feature would provide greater support for attrs and allow for defining fields that are initialized & set by converting from other types.

Empieza reproduciendo el ejemplo proporcionado de Python con mypy y revisando cómo se infieren actualmente los tipos de los convertidores de attrs. El issue no menciona archivos ni tests específicos, por lo que el trabajo requiere localizar el plugin de attrs y las rutas relevantes de comprobación de asignaciones; estará terminado cuando se acepte la asignación mostrada sin debilitar otras comprobaciones de tipos.