Change "var1 == var2" to "var1 is var2" (#3880)

This is a detail and might be just a matter of taste. My reason for proposing this change is that the Python "is" operator stands for a stronger type of equality than the "==" operator, so perhaps it drives home the equality point more strongly. However, again, this could just be a matter of taste; I will not be offended if this proposal is rejected.