py3: fix sorting of obsolete markers when building bundle

Last item of marker tuple (parents) is either None or tuple. Comparison
thus fails on Python 3 with:

  TypeError: '<' not supported between instances of 'tuple' and 'NoneType'

Fixing this by coercing None to the empty tuple when sorting markers in
exchange._getbundleobsmarkerpart().

mercurial/exchange.py

@@ -2567,7 +2567,8 b' def _getbundleobsmarkerpart('
             heads = repo.heads()
         subset = [c.node() for c in repo.set(b'::%ln', heads)]
         markers = repo.obsstore.relevantmarkers(subset)
-        markers = sorted(markers)
+        # last item of marker tuple ('parents') may be None or a tuple
+        markers = sorted(markers, key=lambda m: m[:-1] + (m[-1] or (),))
         bundle2.buildobsmarkerspart(bundler, markers)