Problem
1. Implement __str__ method so that lists are displayed the Python way (with square brackets).
2. Implement the remaining operations defined in the Unordered List ADT (append, index, pop, insert).
3. Implement a slice method for the Unordered List class. It should take two parameters, start and stop, and return a copy of the list starting at the start position and going up to but not including the stop position.