Add default (None) values to HBChannel constructor

This is needed for PyQt5 compatibility

IPython/kernel/channels.py

@@ -47,7 +47,7 b' class HBChannel(Thread):'
     _pause = None
     _beating = None
 
-    def __init__(self, context, session, address):
+    def __init__(self, context=None, session=None, address=None):
         """Create the heartbeat monitor thread.
 
         Parameters