Removing state pollution in 'data'

[sturmianseq]

This PR aims to improve test reliability of test test_result_item_missing_data by removing state pollution of data by making a deepcopy.

The test can fail in this way by running pip3 install pytest-repeat; python3 -m pytest --count=2 tests/test_media_art.py::test_result_item_missing_data:

    async def test_result_item_missing_data():
        """Test Parsing with missing keys returns None."""
        data = MOCK_DATA
>       data.pop("gameContentTypesList")
E       KeyError: 'gameContentTypesList'

tests/test_media_art.py:335: KeyError

It may be better to clean state pollutions so that some other tests won't fail in the future due to the shared state pollution.