Add support for classmethod converters in attrs plugin

[tombo315]

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.

[ilevkivskyi]

Sounds like a reasonable idea.

[alexchandel]

Surprised this doesn't already work. Class methods are the best way to construct many types, and are recommended by attrs. Is it especially hard to do this?

[ikonst]

Superficially looking at the code, it appears that every "case" has very specific code handling it. I haven't dug deep to understand why it's this way.

[erictraut]

If PEP 712 is accepted and implemented in mypy, this functionality will come "for free" because attrs uses dataclass_transform.