fetchone() does not follow pep-0249

the fetchone() method of cursor raises "InterfaceError: Fetched all rows already." when called with no further rows remaining. It should be returning None as per pep-0249:

  .fetchone()
    Fetch the next row of a query result set, returning a
    single sequence, or None when no more data is
    available. [6]

Test case:

    from mysql import connector

    conn = connector.connect(user='scott', password='tiger', database='test', host='localhost')

    cursor = conn.cursor()
    cursor.execute("select 1")

    # succeeds
    assert cursor.fetchone() == (1,)

    # fails
    assert cursor.fetchone() is None