Python problem randomization example

@srpearce @JAAkana 

I will duplicate in OLX guide after review and changes
This is a good place to include https://openedx.atlassian.net/browse/DOC-264, "randomization has a 20-seed limit". I've had conversations about this with course teams that anticipated that every student would receive unique numbers if the problem had a broad enough random range, and were puzzled when certain numbers turned up repeatedly during testing. It was never a dealbreaker (20 seeds felt like enough to discourage cheating), but is relevant if someone's doing serious analytics/research. 

(Some of the design intent behind 20 seeds is here: https://openedx.atlassian.net/browse/PR-251)