Inadvertent ipywidgets dependency?

[jsignell]

I pip installed streamz and am seeing an unstated dependency on ipywidgets:

---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
~/conda/envs/2019-06-12_0/lib/python3.6/site-packages/streamz/core.py in _ipython_display_(self, **kwargs)
    259         try:
--> 260             from ipywidgets import Output
    261             import IPython

ModuleNotFoundError: No module named 'ipywidgets'

During handling of the above exception, another exception occurred:

AttributeError                            Traceback (most recent call last)
~/conda/envs/2019-06-12_0/lib/python3.6/site-packages/IPython/core/formatters.py in __call__(self, obj)
    916             method = get_real_method(obj, self.print_method)
    917             if method is not None:
--> 918                 method()
    919                 return True
    920

~/conda/envs/2019-06-12_0/lib/python3.6/site-packages/streamz/collection.py in _ipython_display_(self, **kwargs)
    231     def _ipython_display_(self, **kwargs):
    232         return self.stream.latest().rate_limit(
--> 233             0.5).gather()._ipython_display_(**kwargs)
    234
    235     def emit(self, x):

~/conda/envs/2019-06-12_0/lib/python3.6/site-packages/streamz/core.py in _ipython_display_(self, **kwargs)
    261             import IPython
    262         except ImportError:
--> 263             return self._repr_html_()
    264         output = Output(_view_count=0)
    265         output_ref = weakref.ref(output)

AttributeError: 'rate_limit' object has no attribute '_repr_html_'