QT/NavidationToolbar2: configure subplots dialog should be modal.

Fix for a case when parent windows is modal. Configure subplots hides behind parent modal windows in case of non-modal usage.
Other dialogs of NavigationToolbarQT are modal, so it looks like a bug.

tested on pyQT6

Author: pavelliavonau

lib/matplotlib/backends/backend_qt.py

@@ -759,6 +759,7 @@ def configure_subplots(self):
             self.canvas.mpl_connect(
                 "close_event", lambda e: self._subplot_dialog.reject())
         self._subplot_dialog.update_from_current_subplotpars()
+        self._subplot_dialog.setModal(True)
         self._subplot_dialog.show()
         return self._subplot_dialog