title: dlib cnn_face_detection_model_v1 Python error
body: <p>I'm trying to use the dlib (v19.6) Python API to create a CNN face detector using the code: </p>
  
  <pre><code>cnn_face_detector = dlib.cnn_face_detection_model_v1('mmod_human_face_detector.dat')
  </code></pre>
  
  <p>However, I get an <code>ArgumentError</code> as follows:</p>
  
  <pre><code>---------------------------------------------------------------------------
  ArgumentError                             Traceback (most recent call last)
  &lt;ipython-input-16-c2ca0a6e8dff&gt; in &lt;module&gt;()
  ----&gt; 1 cnn_face_detector = dlib.cnn_face_detection_model_v1('mmod_human_face_detector.dat')
  
  ArgumentError: Python argument types in
      cnn_face_detection_model_v1.__init__(cnn_face_detection_model_v1, str)
  did not match C++ signature:
      __init__(_object*, std::__cxx11::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt;)
  </code></pre>
  
  <p>What might I be doing wrong? Can I not pass the filename of the model file simply as a string?</p>