title: HTML Test runner giving error to import stringIO even after importing it
body: <p>Hi I'm using the following code to implement HTML test runner in my code:</p>
  
  <pre><code>import HtmlTestRunner
  import unittest
  from io import StringIO
  
  class TestStringMethods(unittest.TestCase):
      """ Example test for HtmlRunner. """
  
      def test_upper(self):
          self.assertEqual('foo'.upper(), 'FOO')
  
      def test_isupper(self):
          self.assertTrue('FOO'.isupper())
          self.assertFalse('Foo'.isupper())
  
      def test_split(self):
          s = 'hello world'
          self.assertEqual(s.split(), ['hello', 'world'])
          # check that s.split fails when the separator is not a string
          with self.assertRaises(TypeError):
              s.split(2)
  
      def test_error(self):
          """ This test should be marked as error one. """
          raise ValueError
  
      def test_fail(self):
          """ This test should fail. """
          self.assertEqual(1, 2)
  
      @unittest.skip("This is a skipped test.")
      def test_skip(self):
          """ This test should be skipped. """
          pass
  
  if __name__ == '__main__':
      unittest.main(testRunner=HtmlTestRunner.HTMLTestRunner(output='test_dir'))
  </code></pre>
  
  <p>but getting the  following error:</p>
  
  <blockquote>
    <p>C:\Users\inswadhwa\AppData\Local\Programs\Python\Python36-32‌​\python.exe C:/Users/inswadhwa/PycharmProjects/automation/assertion.py Traceback (most recent call last): File "C:/Users/inswadhwa/PycharmProjects/automation/assertion.py"‌​, line 2, in  import HTMLTestRunner File "C:\Users\inswadhwa\AppData\Local\Programs\Python\Python36-3‌​2\lib\HTMLTestRunner‌​.py", line 97, in  import StringIO ModuleNotFoundError: No module named 'StringIO' Process finished with exit code 1</p>
  </blockquote>
  
  <p>I have already imported <code>StringIO</code>.</p>
  
  <p>Can anyone please suggest a way to overcome the issue?</p>