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.

It's fairly common for a Python script to allow a user to exit using `ctrl + d` or `ctrl + c`. Preventing this may confuse and frustrate a user.