fix(__init__): only import Broker for python>2

The broker uses asyncio, which does not exist in python2. For python2
scripts to use the manager, make the broker import in __init__ depending
on the python (major) version.

Author: nritsche

comet/__init__.py

@@ -1,4 +1,9 @@
 """CoMeT: A Config and Metadata Tracker."""
 from .version import __version__
-from .broker import Broker
 from .manager import Manager, ManagerError, BrokerError, CometError
+
+# The broker uses asyncio which doesn't exist in python2. However we want to be able to use the
+# manager in python2, so make the Broker import dependent on the python version.
+from sys import version_info
+if version_info.major > 2:
+    from .broker import Broker