updated deprecated cgi.parse_qsl to use six's parse_qsl

According to Python 2 documentation: https://docs.python.org/2/library/cgi.html#cgi.parse_qsl

>"This function is deprecated in this module. Use urlparse.parse_qsl() instead. It is maintained here only for backward compatiblity."

For future compatibility with Python 3, the `scrapy.utils.url` module is updated to use `six` library's `urllib.parse.parse_qsl` instead.
https://pythonhosted.org/six/#module-six.moves.urllib.parse