title: Ipython widget Dropdown including NaN values
body: <p>I have an ipython widget dropdown which includes NaN:</p>
  <pre><code>from ipywidgets import widgets
  import numpy as np
  widgets.Dropdown(description='Choose this:', options=[None, 'a', 'b', np.nan])
  </code></pre>
  <p>When I try to select NaN, I get the following error:</p>
  <pre><code>StopIteration                             Traceback (most recent call last)
  ...
  
  During handling of the above exception, another exception occurred:
  
  ValueError                                Traceback (most recent call last)
  ...
  ValueError: nan not in array
  
  During handling of the above exception, another exception occurred:
  
  TraitError                                Traceback (most recent call last)
  ...
  TraitError: Invalid selection: value not found
  </code></pre>
  <p>I removed the Tracebacks because the bot is forcing me to add more details because my post is &quot;mostly code&quot;.</p>
  <p>At first I thought we can only have literals in the dropdown, but using objects generally works fine.
  What is so peculiar about NaN?</p>