that this structure is only intended to track additions
and removals from the dictionary, not recursive tracking
of embedded changes.  fixes #3646.

(cherry picked from commit 5497b1086f9fe44e0594079b036ea939fbc3c479)

If a class inherited from MutableDict (say, for instance, to add an update() method), coerce() would give back an instance of MutableDict instead of an instance of the derived class.

  to get all flake8 passing

report change events for the ``setdefault()`` dictionary operation.
fixes #3051

:func:`.attributes.flag_modified` where the change event would not be
propagated if the attribute had been reassigned to itself.
fixes #2997

This had me reread the code twice to see if I missed why the import was
present.