The following code causes a SystemError:
class BadMapping:
    get = dict.get
    __setitem__ = dict.__setitem__

import _collections
_collections._count_elements(BadMapping(), [42])


This is because _count_elements() (in Modules/_collectionsmodule.c) assumes
that the mapping argument is a dictionary in case it has the same get() and
__setitem__() methods as dict. And so, _count_elements() passes the mapping
argument to _PyDict_GetItem_KnownHash(), which raises the SystemError.


ISTM it is a very unlikely corner case, so that adding a test (as well as
a NEWS.d item) for it is unnecessary.
What do you think?