title: ValueError: Unknown projection '3d' (once again)
body: <p>When executing this line of code:</p>
  <pre><code>import matplotlib.pyplot as plt
  
  #your code
  
  fig = plt.figure()
  ax = fig.gca(projection='3d')
  </code></pre>
  <p>I have an output error:</p>
  <pre><code>raise ValueError(&quot;Unknown projection %r&quot; % projection)
  
  ValueError: Unknown projection '3d'
  
  &lt;Figure size 432x288 with 0 Axes&gt;
  </code></pre>
  <p>The error appears also when I use Spyder as IDE.
  The version of <code>matplotlib</code> is</p>
  <pre><code>print('matplotlib: {}'.format(matplotlib.__version__))
  matplotlib: 1.5.0rc3
  </code></pre>
  <p>But I had the same problem even with other versions of <code>matplotlib</code>.
  A similar error was reported in <a href="https://stackoverflow.com/questions/3810865/matplotlib-unknown-projection-3d-error">this question (Stackoverflow)</a> but the answers do not help. Some suggestions on how to modify the instruction?
  matplotlib: 3.0.2</p>