title: pylint can't find QWidget and QApplication
body: <pre><code>import sys
  from PyQt5.QtWidgets import (QApplication, QWidget)
  app = QApplication(sys.argv)
  window = QWidget()
  window.setGeometry(50, 50, 500, 300)
  window.setWindowTitle('Hello, world')
  window.show()
  sys.exit(app.exec_())
  </code></pre>
  
  <p>I just started to learn pyqt5. I wrote this hello world app, which works. But pylint gives "E0611:No name 'QWidget' in module 'PyQt5.QtWidgets'" and same for QApplication. Is it some kind of bug in pylint?</p>
  
  <pre><code>pylint --version
  No config file found, using default configuration
  pylint 1.7.2, 
  astroid 1.5.3
  Python 3.5.2 (default, Aug 18 2017, 17:48:00) 
  [GCC 5.4.0 20160609]
  </code></pre>