title: MPLBACKEND is ingored
body: <p>The environment variable MPLBACKEND seems to get ignored:</p>
  <pre><code>&gt; MPLBACKEND=QtAgg python -c &quot;import matplotlib.pyplot as plt; print(plt.get_backend())&quot;
  agg
  
  &gt; MPLBACKEND=QtAgg python -c &quot;import matplotlib.pyplot as plt; plt.plot([1],[1]); plt.show()&quot;
  &lt;string&gt;:1: UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure.
  </code></pre>
  <p>Similar with <code>TkAgg</code>. Everything is installed correctly, as setting <code>matplotlib.use('QtAgg')</code> or <code>matplotlib.use('TkAgg')</code> in code works as expected, i.e. it opens a widget with the plot.</p>
  <p>How can I make the environment variable work?</p>
  <p>Edit: A <code>matplotlibrc</code> in the current directory with <code>backend : QtAgg</code> or <code>backend : TkAgg</code> also has no effect.</p>