Add support for marshalling dict-like objects.

[obmarg]

The is_dict_like function in schema.py was just checking if the
object's type was a dict. This causes it to treat any dict subclass as
a model, which fails because dict subclasses are not models.

Updating to check if the item is an instance of collections.Mapping
(or collections.abc.Mapping on python 3) fixes this.

[asottile]

fwiw, I doubt this ImportError will ever be hit (even in python3):

# head -n 8 /usr/lib/python3.5/collections/__init__.py
__all__ = ['deque', 'defaultdict', 'namedtuple', 'UserDict', 'UserList',
            'UserString', 'Counter', 'OrderedDict', 'ChainMap']

# For backwards compatibility, continue to make the collections ABCs
# available through the collections module.
from _collections_abc import *
import _collections_abc
__all__ += _collections_abc.__all__

[analogue]

Will fix the extraneous import logic post-merge.

[analogue]

@obmarg Thanks! 👍