title: Increase thickness in a matplotlib annotation double sided arrow
body: <p>The <code>width</code> parameter works fine for a single sided arrow but I get an error when using a double sided arrow. </p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  x = np.linspace(0, 10, 1000)
  plt.plot(x, np.sin(x));
  plt.annotate('', xy=(3.2, 0), xycoords='data',
               xytext=(5.9, 0), textcoords='data',
               arrowprops=dict(facecolor='black', width=3))
  </code></pre>
  
  <p>This works fine and results in this plot: 
  <a href="https://i.stack.imgur.com/3zEbH.png" rel="noreferrer"><img src="https://i.stack.imgur.com/3zEbH.png" alt="enter image description here"></a></p>
  
  <p>The code also works for a double sided arrow with no width specified:</p>
  
  <pre><code>plt.annotate('', xy=(3.2, 0), xycoords='data',
               xytext=(5.9, 0), textcoords='data',
               arrowprops=dict(facecolor='black', arrowstyle='&lt;-&gt;'))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/JMRah.png" rel="noreferrer"><img src="https://i.stack.imgur.com/JMRah.png" alt="enter image description here"></a></p>
  
  <p>But for a double arrow with width:</p>
  
  <pre><code>plt.annotate('', xy=(3.2, 0), xycoords='data',
               xytext=(5.9, 0), textcoords='data',
               arrowprops=dict(facecolor='black', width=3, arrowstyle='&lt;-&gt;'))
  </code></pre>
  
  <p>results in the matplotlib error: "AttributeError: Unknown property width" </p>