title: Numpy: Checking if a value is NaT
body: <pre><code>nat = np.datetime64('NaT')
  nat == nat
  &gt;&gt; FutureWarning: In the future, 'NAT == x' and 'x == NAT' will always be False.
  
  np.isnan(nat)
  &gt;&gt; TypeError: ufunc 'isnan' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''
  </code></pre>
  
  <p>How can I check if a datetime64 is NaT?  I can't seem to dig anything out of the docs.  I know Pandas can do it, but I'd rather not add a dependency for something so basic.</p>