typing: add an assertion to verify.py to appease pytype

Differential Revision: https://phab.mercurial-scm.org/D10212

mercurial/verify.py

@@ -434,6 +434,7 b' class verifier(object):'
                 filenodes.setdefault(f, {}).update(onefilenodes)
 
         if not dir and subdirnodes:
+            assert subdirprogress is not None  # help pytype
             subdirprogress.complete()
             if self.warnorphanstorefiles:
                 for f in sorted(storefiles):