wx: Fix file extension for toolmanager-style toolbar

Fixes AppVeyor failure noted in
matplotlib#26710 (comment)

Author: QuLogic

lib/matplotlib/backends/backend_wx.py

@@ -1161,6 +1161,8 @@ def set_history_buttons(self):
 # tools for matplotlib.backend_managers.ToolManager:
 
 class ToolbarWx(ToolContainerBase, wx.ToolBar):
+    _icon_extension = '.svg'
+
     def __init__(self, toolmanager, parent=None, style=wx.TB_BOTTOM):
         if parent is None:
             parent = toolmanager.canvas.GetParent()