charmhelpers.core.hookenv fails to import enum in python2

I've filed an issue on github for this: https://github.com/juju/charm-helpers/issues/500

After refreshing charmhelpers on a python2 charm, I ran into issues during import of core.hookenv and core.host due to bare import of enum (available in python3.4+)

...
install Setting up python-six (1.14.0-2) ...
install Traceback (most recent call last):
install File "hooks/hooks.py", line 14, in <module>
install from charmhelpers.core.host import (
install File "/var/lib/juju/agents/unit-rsyslog-forwarder-ha-0/charm/hooks/charmhelpers/core/host.py", line 37, in <module>
install from .hookenv import log, INFO, DEBUG, local_unit, charm_name
install File "/var/lib/juju/agents/unit-rsyslog-forwarder-ha-0/charm/hooks/charmhelpers/core/hookenv.py", line 24, in <module>
install from enum import Enum
install ImportError: No module named enum

I've updated my code to run on python3 to workaround this. Is there a requirement for python3 for v0.20.16?

Is there a stable charm-helpers tagged release that supports py2?