[`dict`] `.popitem()` returns arbitrary element

[LukeSavefrogs]

See this test.

On Python/Jython <3.6 the order of elements in lists/dictionaries is not guaranteed, and in this cases this leads to unexpected results in the original .popitem() method.

For example, the following could happen on Python 2.7-3.5:

test = {"first": 1, "second": 2, "third": 3}

print(test) # Prints {"third": 3, "first": 1, "second": 2}

Should this be kept as intended behaviour (ordered dictionaries were made default in Python 3.6) or should the polyfill conform to the current standard?

[LukeSavefrogs]

Breaking change

Since there are fundamental language differences between 3.x and 2.x which could lead to unexpected results, for now dict.popitem raises NotImplementedError