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.

Beginne damit, das bereitgestellte Python-Beispiel mit mypy zu reproduzieren und zu prüfen, wie die Typen von attrs-Konvertern derzeit inferiert werden. Das Issue nennt keine spezifischen Dateien oder Tests, daher erfordert die Arbeit, die relevanten Pfade im attrs-Plugin und bei der Zuweisungsprüfung zu finden; sie ist abgeschlossen, wenn die gezeigte Zuweisung akzeptiert wird, ohne andere Typprüfungen abzuschwächen.