Handle comm messages in execute preprocessor

jupyter_nbconvert/preprocessors/execute.py

@@ -130,6 +130,8 b' class ExecutePreprocessor(Preprocessor):'
             elif msg_type == 'clear_output':
                 outs = []
                 continue
+            elif msg_type.startswith('comm'):
+                continue
 
             try:
                 out = output_from_msg(msg)