Fix Qt enum access.

This was missed in the global fixing of enums for PyQt6 (now using
_enum everywhere).  To trigger, try to interactively save a figure but
input an invalid format, e.g. "foo.bar".

Author: anntzer

lib/matplotlib/backends/backend_qt.py

@@ -799,7 +799,8 @@ def save_figure(self, *args):
             except Exception as e:
                 QtWidgets.QMessageBox.critical(
                     self, "Error saving file", str(e),
-                    QtWidgets.QMessageBox.Ok, QtWidgets.QMessageBox.NoButton)
+                    _enum("QtWidgets.QMessageBox.StandardButton").Ok,
+                    _enum("QtWidgets.QMessageBox.StandardButton").NoButton)
 
     def set_history_buttons(self):
         can_backward = self._nav_stack._pos > 0