title: matplotlib pyqt5 backend issue
body: <p>I was try to embed matplotlib into pyqt5, my environment is window7 with python3.4, matplotlib 1.5 (matplotlib-1.5.0rc2.win-amd64-py3.4.exe).</p>
  
  <p>I found a example show it work on <a href="http://www.boxcontrol.net/embedding-matplotlib-plot-on-pyqt5-gui.html#.Vh9eBd-qqkp" rel="nofollow">http://www.boxcontrol.net/embedding-matplotlib-plot-on-pyqt5-gui.html#.Vh9eBd-qqkp</a> and the environment is almost the same as mine, except the matplotlib is Matplotlib 1.4.2</p>
  
  <p>however when I import backend_qt5agg, exception was raising: </p>
  
  <blockquote>
    <p>File "C:\Python34\lib\site-packages\matplotlib\backends\backend_qt5agg.py", line 15, in <br>
        from .backend_qt5 import QtCore<br>
    File "C:\Python34\lib\site-packages\matplotlib\backends\backend_qt5.py", line 27, in <br>
        import matplotlib.backends.qt_editor.figureoptions as figureoptions
    File "C:\Python34\lib\site-packages\matplotlib\backends\qt_editor\figureoptions.py", line 17, in 
        import matplotlib.backends.qt_editor.formlayout as formlayout<br>
    File "C:\Python34\lib\site-packages\matplotlib\backends\qt_editor\formlayout.py", line 58, in <br>
        from matplotlib.backends.qt_compat import QtGui, QtWidgets, QtCore<br>
    File "C:\Python34\lib\site-packages\matplotlib\backends\qt_compat.py", line 144, in <br>
        raise RuntimeError("PyQt{4,5} bindings found despite sip importing\n"<br>
        RuntimeError: PyQt{4,5} bindings found despite sip importing
    Please install PyQt4 or PyQt5, uninstall sip or explicitly set the pyside backend.</p>
  </blockquote>
  
  <pre><code>import matplotlib
  matplotlib.use("Qt5Agg", force=True)
  
  from matplotlib.figure import Figure
  
  from matplotlib.backends.backend_qt5agg import (
      FigureCanvasQTAgg as FigureCanvas,
      NavigationToolbar2QT as NavigationToolbar)
  </code></pre>
  
  <p>I am seperately using Pyqt5 and matplotlib in my other project, and both work.
  Could you tell me how to fix it?</p>
  
  <p><strong>Update</strong> after I uninstall matplotlib matplotlib-1.5.0, and install 1.4.2 instead, the above imports don't throw exception. Would that be a bug of 1.5.2</p>