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.

Start by reproducing the provided Python example with mypy and reviewing how attrs converter types are currently inferred. The issue mentions no specific files or tests, so the work requires locating the relevant attrs plugin and assignment-checking paths; it is done when the shown assignment is accepted without weakening other type checks.