Debugger: Can't clean for JSON Error

Hi,

I am seeing the following error on master.

Create a notebook with:
```
class Foo(object):
pass
Foo()
```

Run the cell. Then try to start the debugger. It fails with this in stdout of jupyter:
```
[IPKernelApp] ERROR | Exception in control handler:
Traceback (most recent call last):
File "/path/to/lib64/python3.7/site-packages/ipykernel/kernelbase.py", line 243, in process_control
yield gen.maybe_future(handler(self.control_stream, idents, msg))
File "/path/to/lib64/python3.7/site-packages/tornado/gen.py", line 762, in run
value = future.result()
File "/path/to/lib64/python3.7/site-packages/tornado/gen.py", line 775, in run
yielded = self.gen.send(value)
File "/path/to/lib64/python3.7/site-packages/ipykernel/kernelbase.py", line 757, in debug_request
reply_content = json_clean(reply_content)
File "/path/to/lib64/python3.7/site-packages/ipykernel/jsonutil.py", line 142, in json_clean
out[str(k)] = json_clean(v)
File "/path/to/lib64/python3.7/site-packages/ipykernel/jsonutil.py", line 142, in json_clean
out[str(k)] = json_clean(v)
File "/path/to/lib64/python3.7/site-packages/ipykernel/jsonutil.py", line 128, in json_clean
return [json_clean(x) for x in obj]
File "/path/to/lib64/python3.7/site-packages/ipykernel/jsonutil.py", line 128, in
return [json_clean(x) for x in obj]
File "/path/to/lib64/python3.7/site-packages/ipykernel/jsonutil.py", line 142, in json_clean
out[str(k)] = json_clean(v)
File "/path/to/lib64/python3.7/site-packages/ipykernel/jsonutil.py", line 148, in json_clean
raise ValueError("Can't clean for JSON: %r" % obj)
ValueError: Can't clean for JSON:
```