Stubs: rename members of a nested companion (PathOperation.Companion.reverseDifference) #86

scripts/gen_stubs.py does not rename the members of a companion nested inside a class: the committed ui/graphics stub still spells PathOperation.Companion.reverseDifference in camelCase, while every other member is snake_case. It predates #79 (found during its step 5). Done when nested companion members follow python_name like every other member, a generator test covers a nested companion, the stubs are regenerated, and the runtime spelling is checked to match (the binder's own alias for that member, python-multiplatform #131).