instances of UserDict.UserDict are not iterable
but instances of collections.UserDict are iterable

Fixes #351.

…lib.request (#388)

python/cpython#84850

> Remove URLopener and FancyURLopener classes from urllib.request.
> They had previously raised DeprecationWarning since Python 3.3.

The test failures were:

    __________________ test_move_items_urllib_request[URLopener] ___________________

    item_name = 'URLopener'

        @pytest.mark.parametrize("item_name",
                                  [item.name for item in six._urllib_request_moved_attributes])
        def test_move_items_urllib_request(item_name):
            """Ensure that everything loads correctly."""
            assert item_name in dir(six.moves.urllib.request)
    >       getattr(six.moves.urllib.request, item_name)
    E       AttributeError: module 'six.moves.urllib.request' has no attribute 'URLopener'

    test_six.py:170: AttributeError
    ________________ test_move_items_urllib_request[FancyURLopener] ________________

    item_name = 'FancyURLopener'

        @pytest.mark.parametrize("item_name",
                                  [item.name for item in six._urllib_request_moved_attributes])
        def test_move_items_urllib_request(item_name):
            """Ensure that everything loads correctly."""
            assert item_name in dir(six.moves.urllib.request)
    >       getattr(six.moves.urllib.request, item_name)
    E       AttributeError: module 'six.moves.urllib.request' has no attribute 'FancyURLopener'

    test_six.py:170: AttributeError