Initialize new save_warnings_filters data member in ModifiedInterpreter.__init__.

Was getting mystery
    ModifiedInterpreter instance has no attribute 'save_warnings_filters'
errors at odd times (for sure in a fresh IDLE, fresh file, then Run Script).

Tools/idle/PyShell.py

@@ -157,6 +157,7 @@ class ModifiedInterpreter(InteractiveInterpreter):
         self.tkconsole = tkconsole
         locals = sys.modules['__main__'].__dict__
         InteractiveInterpreter.__init__(self, locals=locals)
+        self.save_warnings_filters = None
 
     gid = 0