Update for new nbconvert

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For nbconvert 6 (and jupyter_client, nbclient, etc)

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See builds of #44 for example problems, which were fixed by

diff --git a/setup.py b/setup.py
index dedb612..c08ccd8 100644
--- a/setup.py
+++ b/setup.py
@@ -52,7 +52,7 @@ setup_args = dict(
         # * Optional: process ipython magics (required import: IPython)
         'jupyter_client',
         'nbformat',
-        'nbconvert',
+        'nbconvert <6',
     # TODO: I think the decision was to go with python/setup.py for this stuff,
     # right? (but if so, how do I specify it's the runtime python version
     # I'm talking aobut, not the buildtime python version?)

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Errors look like

Error in atexit._run_exitfuncs:
Traceback (most recent call last):
  File "/home/travis/build/pyviz-dev/nbsmoke/.tox/py37/lib/python3.7/site-packages/traitlets/traitlets.py", line 535, in get
    value = obj._trait_values[self.name]
KeyError: 'session'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/home/travis/build/pyviz-dev/nbsmoke/.tox/py37/lib/python3.7/site-packages/nbclient/client.py", line 358, in _async_cleanup_kernel
    await ensure_async(self.km.cleanup_resources())
  File "/home/travis/build/pyviz-dev/nbsmoke/.tox/py37/lib/python3.7/site-packages/jupyter_client/manager.py", line 355, in cleanup_resources
    self.session.parent = None
  File "/home/travis/build/pyviz-dev/nbsmoke/.tox/py37/lib/python3.7/site-packages/traitlets/traitlets.py", line 575, in __get__
    return self.get(obj, cls)
  File "/home/travis/build/pyviz-dev/nbsmoke/.tox/py37/lib/python3.7/site-packages/traitlets/traitlets.py", line 548, in get
    value = self._validate(obj, default)
  File "/home/travis/build/pyviz-dev/nbsmoke/.tox/py37/lib/python3.7/site-packages/traitlets/traitlets.py", line 610, in _validate
    value = self.validate(obj, value)
  File "/home/travis/build/pyviz-dev/nbsmoke/.tox/py37/lib/python3.7/site-packages/traitlets/traitlets.py", line 1844, in validate
    self.error(obj, value)
  File "/home/travis/build/pyviz-dev/nbsmoke/.tox/py37/lib/python3.7/site-packages/traitlets/traitlets.py", line 690, in error
    raise TraitError(e)
traitlets.traitlets.TraitError: The 'session' trait of an AsyncKernelManager instance expected a Session, not the Session at '0x7fcc8a64dbe0'.```

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#48 fixed