isogram: Update assertFalse with assertIs to avoid false positives 
isogram: Update assertFalse with assertIs to avoid false positives 


N-Parsons/exercism-python,behrtam/xpython,jmluy/xpython,behrtam/xpython,N-Parsons/exercism-python,pheanex/xpython,mweb/python,mweb/python,exercism/xpython,smalley/python,exercism/python,pheanex/xpython,exercism/python,exercism/xpython,jmluy/xpython,smalley/python