Merge pull request kivy#2662 from kivy/pygame_scr

Window.screenshot python 3 fix

Author: akshayaurora

kivy/core/window/window_pygame.py

@@ -260,7 +260,10 @@ def screenshot(self, *largs, **kwargs):
                                               GL_UNSIGNED_BYTE)
         width, height = self.system_size
         data = glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE)
-        data = str(buffer(data))
+        if PY2:
+            data = str(buffer(data))
+        else:
+            data = bytes(bytearray(data))
         surface = pygame.image.fromstring(data, (width, height), 'RGBA', True)
         pygame.image.save(surface, filename)
         Logger.debug('Window: Screenshot saved at <%s>' % filename)