typing: add an assertion to util.versiontuple

Fixes the following warning:

    line 1177, in versiontuple: No attribute 'split' on None [attribute-error]
         In Optional[bytes]

Differential Revision: https://phab.mercurial-scm.org/D7676

mercurial/util.py

@@ -1173,6 +1173,8 b' def versiontuple(v=None, n=4):'
     else:
         vparts, extra = m.group(1), None
 
+    assert vparts is not None  # help pytype
+
     vints = []
     for i in vparts.split(b'.'):
         try: