title: Add type hint to variable mask
body: <p>I've just started with Python (v.3.7.7) and with PyCharm.</p>
  <p>With this code:</p>
  <pre><code>class NiftiImages:
      def __init__(self):
          self.brain = None
          self.mask = None
  </code></pre>
  <p>PyCharm suggest me to &quot;Add type hint to variable 'mask'. And if I hit it, I get this:</p>
  <pre><code>class NiftiImages:
      mask: None
  
      def __init__(self):
          self.brain = None
          self.mask = None
  </code></pre>
  <p><code>brain</code> and <code>mask</code> will always be <code>Numpy Array</code>.</p>
  <p>What is a type hint for a variable (in this case for a <code>Numpy Array</code>)?</p>