Assignment 3 - List Lab and ROT13

Per [PEP 8](https://www.python.org/dev/peps/pep-0008/#maximum-line-length), maximum line length is 79 characters. One way around this is to use "Python's implied line continuation inside parentheses, brackets and braces." However, I tend to prefer using intermediate symbols over implied line continuation when there are operators involved.

These tests are sufficient for this assignment. However, I recommend that, whenever possible, you include tests that are easily verified by a person reviewing your code. This will provide the reviewer with more confidence that the more complex tests for which you supply both the input and the desired output are valid. An example for this assignment would be to start with a test that asserts something like:

    assert rot13(first_half_of_alphabet) == (second_half_of_alphabet)