util: eliminate the possibility of returning None from `versiontuple()`

This fixes the following error flagged by pytype:

    File "/mnt/c/Users/Matt/hg/mercurial/extensions.py", line 228, in load: unsupported operand types for > [unsupported-operands]
      Primitive types 'Optional[tuple]' and 'curver: Optional[tuple]' are not comparable.

Differential Revision: https://phab.mercurial-scm.org/D11473

mercurial/util.py

@@ -1225,6 +1225,8 b' def versiontuple(v=None, n=4):'
     if n == 4:
         return (vints[0], vints[1], vints[2], extra)
 
+    raise error.ProgrammingError(b"invalid version part request: %d" % n)
+
 
 def cachefunc(func):
     '''cache the result of function calls'''