Avoid DeprecationWarning about ABC in Python 3.x

[nicktimko]

Python 3.x complains about the collections.Mapping vs collections.abc.Mapping. There is already a shim for it in marshmallow.compat, so switched to using that.

/usr/lib/python3.7/site-packages/marshmallow/marshalling.py:253: DeprecationWarning: Using or importing the ABCs from collections instead of from collections.abc is deprecated, and in 3.8 it will stop working

    if not isinstance(data, collections.Mapping):

Though I poked at the collections.Mapping in 3.8 and it said "...is deprecated since Python 3.3,and in 3.9 it will stop working", so it's maybe not as critical, but it removes nagging. See python/cpython#13409

[sloria]

Thanks!