Mask top level import of `.hilevel`

Since it isn't required until the landing of the new service-manager
stuff in #12; was an oversight
from commit `0607a31dddeba032a2cf7d9fe605edd9d7bb4846`.

tractor/__init__.py

@@ -67,4 +67,4 @@ from ._root import (
 from ._ipc import Channel as Channel
 from ._portal import Portal as Portal
 from ._runtime import Actor as Actor
-from . import hilevel as hilevel
+# from . import hilevel as hilevel