Support classmethod and staticmethod as converters in attrs plugin

[mondeja]

Adds support for passing staticmethod / classmethod with no decorators to converter in attrs.

Is sound only when the member access is the descriptor's canonical counit (the evaluation of its declared signature for classmethod, the identity for staticmethod) i.e. when there are no other decorators, since there the inferred type coincides by construction with the type checker's.

---

I'm currently writing

def _to_foo(value: Any) -> Foo:
    ...

@attrs.define
class MyAttrClass:
    foo: Foo = field(
        factory=Foo,
        converter=_to_foo,
    )

but I would prefer just

@attrs.define
class MyAttrClass:
    foo: Foo = field(
        factory=Foo,
        converter=Foo.from_value,
    )

as from_value is a real converter factory.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅