Don't use the deprecated wx.PySimpleApp class

Fix this warning:

./PoPiPaint.py:28: wxPyDeprecationWarning: Using deprecated class PySimpleApp.
  wx.PySimpleApp.__init__(self, *args, **kwargs)

diff --git a/PoPiPaint.py b/PoPiPaint.py
index 090126f..683f2a1 100755 (executable)
--- a/PoPiPaint.py
+++ b/PoPiPaint.py
@@ -22,10 +22,10 @@ import wx
 import CanvasFrame
 
 
-class PoPiPaApp(wx.PySimpleApp):
+class PoPiPaApp(wx.App):
     def __init__(self, *args, **kwargs):
         self.base_image = kwargs.pop('base_image', None)
-        wx.PySimpleApp.__init__(self, *args, **kwargs)
+        wx.App.__init__(self, *args, **kwargs)
 
     def OnInit(self):
         # We do not want a resizeable frame!