[devtools] fix get_bounds Say an attribute has two IntervalBoundConstraints, one with minvalue=0 and the other with maxvalue=10 (not literally, or that could be a single constraint, but e.g. with references to other attributes). We don't want to override the minvalue of the former with None just because the latter happens last in the constraints list. This doesn't fix the case where two constraints both have a min or a max value. Meh.