title: Error while saving a matplotlib animation, missing 'dpi' argument
body: <p>I'm trying to save an animation of matplotlib.animation.AnimationFunc and I get an error saying 'dpi' argument missing. Obviously, I have the dpi set so I don't understand where this error comes from.</p>
  
  <p>I'm running python 3.6 and matplotlib 3.0.3, I also just installed ffmpeg from ubuntu official repositories (Ubuntu 18.04).</p>
  
  <p>This is the part of my code that should affect that, although I think it should be something of the system:</p>
  
  <pre><code>Writer = writers['ffmpeg']
  writer = Writer(fps=15, metadata=dict(artist='Me'), bitrate=1800,)
  ani = FuncAnimation(fig, anime, interval=time_step *
                      10**3, frames=F, repeat=False,) 
  ani.save('standard_map.mp4', writer=Writer, dpi=100)
  </code></pre>
  
  <p>The errors is:</p>
  
  <pre><code>with writer.saving(self._fig, filename, dpi):
  File "/usr/lib/python3.6/contextlib.py", line 159, in helper
      return _GeneratorContextManager(func, args, kwds)
  File "/usr/lib/python3.6/contextlib.py", line 60, in __init__
      self.gen = func(*args, **kwds) TypeError: saving() missing 1 required positional argument: 'dpi'
  </code></pre>
  
  <p>I tried both adding the lines they suggested there and the error stills the same. </p>
  
  <pre><code>plt.rcParams['animation.ffmpeg_path'] = '/usr/bin/ffmpeg'
  </code></pre>
  
  <p>I also tried changing the writer to 'imagemagick' the one set on Ubuntu by default and the error persists.</p>