Fix: positional argument specifiers are required by Python 2.6

Author: cgohlke

lib/matplotlib/backends/backend_wx.py

@@ -1254,7 +1254,7 @@ def _get_key(self, evt):
                              [evt.AltDown, 'alt'],
                              [evt.ControlDown, 'ctrl'], ):
             if meth():
-                key = '{}+{}'.format(prefix, key)
+                key = '{0}+{1}'.format(prefix, key)
 
         return key