Make parent class initalization easy

In some cases it is useful to create an new object by copying some of the attributes that are provided by a shared base class. Here is an example

import attr

@attr.s()
class Base(object):
    uid = attr.ib()
    name = attr.ib()
    
@attr.s()
class AreaChild(Base):
    area = attr.ib()
    
@attr.s()
class LengthChild(Base):
    length = attr.ib()
    
def calc_square(length_obj):
    def make_class_filter(cls):
        cls_fields = attr.fields_dict(cls)
        return lambda a,v: a.name in cls_fields

    base_dict = attr.asdict(length_obj, 
                            filter = make_class_filter(Base))
    
    return AreaChild(area = length_obj.length**2, 
                     **base_dict)

lc = LengthChild(uid=42, name='rope', length=3)
print(lc)
ac = calc_square(lc)
print(ac)

As one can see extraction of the base class attributes is somewhat clunky. The calc_square function could be much easier to read if a base class could be passed as a filter to .asdict:

def calc_square(length_obj):
    base_dict = attr.asdict(length_obj, filter = Base)
    return AreaChild(area=length_obj.length**2, **base_dict)

If you would think this makes sense, I could provide a pull request to enable classes as filters with the functionality as described above.

Start by reviewing attrs.asdict and its existing filter behavior, using the Base, LengthChild, and AreaChild example in the issue as the reference case. The work is done when a class such as Base can be passed directly as the filter and produces the same base-attribute dictionary shown in the example, with coverage for that behavior.