typing: add type hints to `mercurial.verify._normpath()`

Since 10db46e128d4, pytype almost figured this out, going from `Any` -> `_T0`,
but the intent is obvious.

mercurial/verify.py

@@ -31,7 +31,7 b' def verify(repo, level=None):'
         return v.verify()
 
 
-def _normpath(f):
+def _normpath(f: bytes) -> bytes:
     # under hg < 2.4, convert didn't sanitize paths properly, so a
     # converted repo may contain repeated slashes
     while b'//' in f: