Add discussion of integer division to Intermediate Python Intro

1. Fix the integer division typo in the temperature conversion
2. Add discussion of why we need a float in the division for things to work properly

I had originally planned on just slipping the 9.0 in and having the instructors
respond to student question about it, but I think it's worth going ahead and
actively introducing this early following @gvwilson's lead in the novice material.
This is a common hangup with Python so we might as well introduce it quickly
as we're introducing the language more generally.

Thanks to @wking for finding the typo and helping me think about how to
address the integer division issue.