Support a small amount of generic converters for attrs - #22206

# Crash: zip(..., strict=True) in Attribute.argument raises ValueError
from typing import Generic, TypeVar

from attrs import define, field
from typing_extensions import TypeVarTuple, Unpack

T = TypeVar("T")
Ts = TypeVarTuple("Ts")

class V(Generic[Unpack[Ts]]):
    def __init__(self, *x: Unpack[Ts]) -> None: ...

def foo(x: T) -> V[T]: ...

@define
class B:
    v: V[int, str] = field(converter=foo)

from typing import Generic, TypeVar

from attrs import define, field

T = TypeVar("T")
S = TypeVar("S")

def ident(x: T) -> T:
    return x

# The field type S is not an Instance, so the new mapping is skipped.
@define
class A(Generic[S]):
    b: S = field(converter=ident)

# master reveals "b: S", this PR "b: T"
reveal_type(A.__init__)

from typing import Generic, TypeVar

from attrs import define, field

T = TypeVar("T")

class G(Generic[T]):
    def __init__(self, x: T) -> None: ...

@define
class C:
    c: G[int] = field(converter=G)

# Fix lost in subclasses: inherited attributes are rebuilt via
# Attribute.deserialize, which doesn't restore Converter.ret_type.
@define
class E(C):
    e: int = 0

# C gets "c: int" but E gets "c: T" (unbound), so every argument is rejected with
# 'expected "T"', even a valid E(1).
reveal_type(C.__init__)
reveal_type(E.__init__)
E("x")