typing: make `unionrepository` subclass `localrepository` while type checking This is the same change as 9d4ad05bc91c made for `bundlerepository`, for the same reasons. Also, add a comment here to suppress the PyCharm warning that the superclass constructor is not called, that is new now that there's a simulated superclass. That lack of a call is by design- `makeunionrepository()` does magic that PyCharm isn't aware of. But PyCharm has been better at catching problems than pytype in a lot of cases, so I'd like to reduce the bogus things it flags, to make the real issues stand out.