A variant call over a big override family ran out of dispatch slots

Vloot: self.page.update() where Page has nine overriding views, plus dict, set and tkinter's Widget declaring update. The program was refused: "too many candidate classes to dispatch at runtime".

Cause: the candidate scan in PyParseVariantMethod collapses a subclass into its ancestor only when the ancestor is the current hit. With an unrelated class hit first (Widget), every sibling override became a candidate of its own.

Fix: a class whose parent also answers to the name is skipped. The parent is visited in its own turn, and a call through its VMT slot reaches the override. One override family is one candidate.

Test: test_nilpy_dispatch_over_an_override_family (an unrelated Clock first, then Page and 18 views; the pin refuses it).