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.

Start by reproducing the reported error with the classmethod converter example in the issue, then trace the attrs plugin's converter handling. Done means classmethod converters such as Bar.make are accepted without the unsupported-converter error and the example remains correctly type-checked.