Removed is from boolean operators (thw-python refinements)

This PR removes the examples of `is` from the Boolean operators section.  (I don't think anyone will be using is for flow control).  To the discussion of a=b creating two pointers to the same object, it adds an explanation of how to create copies of lists and dicts with `list()` and `dict()`.  

I hope this change 1) removes unnecessary detail from Boolean and 2) provides a solution to a problem, instead of warning about the problem and walking off.