Instead of using assertions to make sure a Circle object isn't instantiated with a zero or negative radius or diameter, used and if statement with a raise ValueError.
Updated test module for circle.py to test that the ValueError is raised in the case of an attempted instantiation of a Circle object with a negative or zero radius (or diameter), instead of the AssertionError.
Merge branch 'master' of https://github.com/UWPCE-PythonCert/IntroToPython
Merge pull request #83 from imdavis/master

Session 07 Assignment: Circle Class