title: matplotlib.figure.Figure.add_subplots() and add_axes() return None instead of axes
body: <p><code>matplotlib.figure.Figure.add_subplots()</code> (<a href="https://matplotlib.org/3.1.0/api/_as_gen/matplotlib.figure.Figure.html?highlight=add_subplot#matplotlib.figure.Figure.add_subplot" rel="nofollow noreferrer">doc</a>) should return an axes. </p>
  
  <p>However, doing</p>
  
  <pre><code>import matplotlib.pyplot as plt
  fig = plt.figure()
  ax = fig.add_subplot()
  print(ax)
  </code></pre>
  
  <p>returns <code>None</code>.</p>
  
  <p>Same happens for <code>ax = fig.add_axes()</code></p>
  
  <p>Why this happens and how can I obtain the axes handle?</p>