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().
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2