using `pip3 install jp_proxy_widget` fails during enable step

[jsoishi]

Using python3.6.6 and jupyter 4.4.0/notebook 5.5.0, if I do

$ pip3 install jp_proxy_widget

it seems to install OK, but when I go to do

$ jupyter nbextension enable --py --sys-prefix jp_proxy_widget

I get

Traceback (most recent call last):
  File "/home/jsoishi/miniconda3/envs/dedalus-kjb/bin/jupyter-nbextension", line
 11, in <module>
    sys.exit(main())
  File "/home/jsoishi/miniconda3/envs/dedalus-kjb/lib/python3.6/site-packages/ju
pyter_core/application.py", line 266, in launch_instance
    return super(JupyterApp, cls).launch_instance(argv=argv, **kwargs)
  File "/home/jsoishi/miniconda3/envs/dedalus-kjb/lib/python3.6/site-packages/tr
aitlets/config/application.py", line 658, in launch_instance
    app.start()
  File "/home/jsoishi/miniconda3/envs/dedalus-kjb/lib/python3.6/site-packages/no
tebook/nbextensions.py", line 988, in start
    super(NBExtensionApp, self).start()
  File "/home/jsoishi/miniconda3/envs/dedalus-kjb/lib/python3.6/site-packages/ju
pyter_core/application.py", line 255, in start
    self.subapp.start()
  File "/home/jsoishi/miniconda3/envs/dedalus-kjb/lib/python3.6/site-packages/no
tebook/nbextensions.py", line 896, in start
    self.toggle_nbextension_python(self.extra_args[0])
  File "/home/jsoishi/miniconda3/envs/dedalus-kjb/lib/python3.6/site-packages/no
tebook/nbextensions.py", line 872, in toggle_nbextension_python
    logger=self.log)
  File "/home/jsoishi/miniconda3/envs/dedalus-kjb/lib/python3.6/site-packages/no
tebook/nbextensions.py", line 483, in enable_nbextension_python
    logger=logger)
  File "/home/jsoishi/miniconda3/envs/dedalus-kjb/lib/python3.6/site-packages/no
tebook/nbextensions.py", line 380, in _set_nbextension_state_python
    m, nbexts = _get_nbextension_metadata(module)
  File "/home/jsoishi/miniconda3/envs/dedalus-kjb/lib/python3.6/site-packages/no
tebook/nbextensions.py", line 1122, in _get_nbextension_metadata
    m = import_item(module)
  File "/home/jsoishi/miniconda3/envs/dedalus-kjb/lib/python3.6/site-packages/tr
aitlets/utils/importstring.py", line 42, in import_item
    return __import__(parts[0])
ModuleNotFoundError: No module named 'jp_proxy_widget'

However, if I instead install via

$ python3 -m pip install jp_proxy_widget

Then everything seems to work OK.