obsutil: defend against succsmarkers() returning None

I'm not sure if this is a realistic problem, but doing this avoids
some pretty awful test failures on Python 3, and it looks like it
should be harmless.

Differential Revision: https://phab.mercurial-scm.org/D274

mercurial/obsutil.py

@@ -307,7 +307,7 b' def getobsoleted(repo, tr):'
         seenrevs.add(rev)
         if phase(repo, rev) == public:
             continue
-        if set(succsmarkers(node)).issubset(addedmarkers):
+        if set(succsmarkers(node) or []).issubset(addedmarkers):
             obsoleted.add(rev)
     return obsoleted