title: FIeld format conversion error in numpy array
body: <p>I am trying to create a numpy array consisting of an array of data, where each data point is another array of length 5. I am trying to do this by setting a first datum (i.e. array of five elements) and setting the name and formats of each data types using the following code:</p>
  <pre><code>DTYPE = [('t_start', 'S32'), ('t_end', 'S32'), ('mac1', 'S32'), ('mac2', 'S32'), ('rss', np.float)]
  
  np.array([ ['2020-10-07 09:20:00', '2020-10-07 09:43:20', 'b8:9a:2a:06:68:f5', 'b8:9a:2a:06:68:f5', '10.501'] ], dtype=DTYPE)
  </code></pre>
  <p>There must be something really obvious I am missing here because this (simple?) code issues the following Exception</p>
  <pre><code>ValueError: could not convert string to float: '2020-10-07 09:20:00'
  </code></pre>
  <p>Any hint on what is going on here would be much appreciated.</p>