Session04

Add 'dict_set.py' file for dict lab assignment.
Add 'trigrams.py' file for trigrams assignment.
Add 'safe_input.py' for exceptions assignment.
Add 'mailroom.py' for mailroom madness assignment.

Nice use of checking for membership! Generally, you'd want a set instead of a list because the time complexity for checking membership in a set is O(1); it's O(n) for lists. However, this list is short enough that the savings would be negligible.