test_configdialog twice patches tkinter.messagebox to mock askyesno without restoring it properly.  As a consequence, the outwin test in #30617 failed because the messagebox module in editor lacked askyesno.

First patch: fix configdialog and test_configdialog.  Similar to what is done elsewhere, make askyesno an instance method so that it is masked by an instance attribute mock and unmasked by deleting the instance function.  In the process, revise the messabebox import,

Follow-on patch(es): check exiting tests and apply the same idea everywhere.  In the process, end use of 'tkMessageBox'.