Add support for classmethod converters in attrs plugin

import attr

@attr.s
class Bar:
    b = attr.ib()

    @classmethod
    def make(cls, one: int) -> 'Bar':
        return cls(one)


@attr.s
class Foo:
    a = attr.ib(converter=Bar.make)

error: Unsupported converter, only named functions and types are currently supported

Attrs is perfectly happy with make

>>> Foo(1)
Foo(a=Bar(b=1))

Granted the converter limitations are a known issue, I couldn't find this use-case covered anywhere.

まず、issue の classmethod converter の例で報告されたエラーを再現し、その後 attrs plugin の converter 処理を追跡します。classmethod converter（Bar.make など）が unsupported-converter エラーなしで受け入れられ、例が引き続き正しく型チェックされれば完了です。