title: Exception in Tkinter callback when using Matplotlib due to permission error
body: <p>I recently set up Ipython on my laptop and wrote this snippet to test the matplotlib installation only to be greeted by this error, the end of the traceback says PermissionError: [Errno 13] Permission denied: 'C:\Users\KARTHIK KC\.matplotlib\fontList.py3k.cache'. I have been unable to resolve this error. Any suggestions?</p>
  
  <pre><code>import matplotlib.pyplot as plt
  x = range(-10, 10,1)
  y = []
  for xi in x:
     yi = xi**2
     y.append(yi)
  plt.plot(x,y)
  plt.show()
  </code></pre>
  
  <p>And this is the error - </p>
  
  <pre><code>Exception in Tkinter callback
  Traceback (most recent call last):
    File "C:\Python34\lib\tkinter\__init__.py", line 1533, in __call__
  return self.func(*args)
    File "C:\Python34\Lib\site-packages\matplotlib\backends\backend_tkagg.py", line 278, in resize
  self.show()
    File "C:\Python34\Lib\site-packages\matplotlib\backends\backend_tkagg.py", line 349, in draw
  FigureCanvasAgg.draw(self)
    File "C:\Python34\Lib\site-packages\matplotlib\backends\backend_agg.py", line 469, in draw
    self.figure.draw(self.renderer)
    File "C:\Python34\Lib\site-packages\matplotlib\artist.py", line 59, in draw_wrapper
  draw(artist, renderer, *args, **kwargs)
    File "C:\Python34\Lib\site-packages\matplotlib\figure.py", line 1085, in draw
  func(*args)
    File "C:\Python34\Lib\site-packages\matplotlib\artist.py", line 59, in draw_wrapper
  draw(artist, renderer, *args, **kwargs)
    File "C:\Python34\Lib\site-packages\matplotlib\axes\_base.py", line 2110, in draw
  a.draw(renderer)
    File "C:\Python34\Lib\site-packages\matplotlib\artist.py", line 59, in draw_wrapper
  draw(artist, renderer, *args, **kwargs)
    File "C:\Python34\Lib\site-packages\matplotlib\axis.py", line 1116, in draw
  renderer)
    File "C:\Python34\Lib\site-packages\matplotlib\axis.py", line 1065, in _get_tick_bboxes
  extent = tick.label1.get_window_extent(renderer)
    File "C:\Python34\Lib\site-packages\matplotlib\text.py", line 798, in get_window_extent
  bbox, info, descent = self._get_layout(self._renderer)
    File "C:\Python34\Lib\site-packages\matplotlib\text.py", line 311, in _get_layout
  ismath=False)
    File "C:\Python34\Lib\site-packages\matplotlib\backends\backend_agg.py", line 232, in get_text_width_height_descent
  font = self._get_agg_font(prop)
    File "C:\Python34\Lib\site-packages\matplotlib\backends\backend_agg.py", line 276, in _get_agg_font
  fname = findfont(prop)
    File "C:\Python34\Lib\site-packages\matplotlib\font_manager.py", line 1421, in findfont
  font = fontManager.findfont(prop, **kw)
    File "C:\Python34\Lib\site-packages\matplotlib\font_manager.py", line 1304, in findfont
  _rebuild()
    File "C:\Python34\Lib\site-packages\matplotlib\font_manager.py", line 1402, in _rebuild
  pickle_dump(fontManager, _fmcache)
    File "C:\Python34\Lib\site-packages\matplotlib\font_manager.py", line 952, in pickle_dump
  with open(filename, 'wb') as fh:
    PermissionError: [Errno 13] Permission denied: 'C:\\Users\\KARTHIK KC\\.matplotlib\\fontList.py3k.cache'
  </code></pre>