topic: drop compat.successorssets(), it's not used
The only place in topics that uses successorssets function is in evolvebits.py,
and it's using that function from obsutil directly.
#!/bin/sh
. $TESTDIR/testlib/pythonpath.sh
# This file holds logic that is used in many tests.
# It can be called in a test like this:
# $ . "$TESTDIR/testlib/topic_setup.sh"
# Enable obsolete markers and enable extensions
cat >> $HGRCPATH << EOF
[experimental]
evolution=createmarkers,exchange
[extensions]
rebase=
topic=
EOF