topic: check availability of obsutil.getmarkers() for portability
Before this patch, topic extension causes unintentional failure with
Mercurial earlier than 4.3, because obsutil.getmarkers() has been
available since Mercurial 4.3 (tests for topic on mercurial-4.*
branches fail, too).
This breaks "minimumhgversion = '4.0'" declaration of topic extension.
This patch fixes this issue in a straightforward way for simplicity on
stable branch.
I'm planning to centralize such portability logic in topic extension
into topic/compat.py or so on default branch for efficiency at
runtime, like as evolve/compat.py.