title: undefined-variable Warning for python 3.x in pylint
body: <p>I am writing a python script but I just faced this issue regarding pylint checks in python 3.x :</p>
  
  <pre><code>class m(object):
      def check_infile(self):
          infile = None
          if not isinstance(infile, file):
              print("infile variable is not a file type.")
  </code></pre>
  
  <p>Output:  </p>
  
  <pre><code>E: 56,38: Undefined variable 'file' (undefined-variable)
  </code></pre>
  
  <p>I tried to eliminate this issue by adding <code># pylint: disable=E0602, undefined-variable, E0603</code> but nothing helps. any suggestion?</p>