title: control.matplotlib.pzmap doesn't show zeros
body: <p>Make a simple transfer function with one zero and two poles and then plot its pole-zero map:</p>
  
  <pre><code>import control
  import matplotlib.pyplot as plt
  tfx = control.tf([1,1],[1,1,1])
  control.matlab.pzmap(tfx)
  plt.show()
  </code></pre>
  
  <p>Result: Python displays a plot with two poles but no zero (python 3.6). 
  Re-do the above with two small modifications as follows:</p>
  
  <pre><code>import control
  import matplotlib.pyplot as plt
  import matplotlib as mpl
  mpl.style.use('classic')
  
  tfx = control.tf([1,1],[1,1,1])
  control.matlab.pzmap(tfx)
  plt.show()
  </code></pre>
  
  <p>Result: Python displays a plot with two poles and one zero.
  My diagnosis: matplotlib default plotting settings does not display circle marker 'o'.
  My problem: I want to use matplotlib default plot settings (Because it's pretty). I DON'T want to use the 'classic' settings. How do I change the default settings to show circular markers? Here is what I've tried:</p>
  
  <pre><code>mpl.rcParams['patch.force_edgecolor'] = True
  mpl.rcParams['patch.edgecolor'] = 'k'
  mpl.rcParams['boxplot.flierprops.markeredgecolor'] = 'k'
  </code></pre>
  
  <p>But it does not change the outcome. Any ideas?</p>
  
  <p>A second and slightly related issue: according to <a href="https://python-control.readthedocs.io/en/latest/generated/control.pzmap.html#control.pzmap" rel="nofollow noreferrer">document on control module</a>, control.pzmap(tfx) instead of control.matlab.pzmap(tfx) should make the pole-zero plot. However that will throw a 'module is not callable' error. Odd...</p>