bpo-42508: Remove bogus idlelib.pyshell.ModifiedInterpreter attribute (GH-23570)

restart_subprocess is a method of self, the pyshell.InteractiveInterpreter instance. The latter does not have an interp attribute redundantly referring to itself. (The PyShell instance does have an interp attribute, referring to the InteractiveInterpreter instance.)
(cherry picked from commit e41bfd15)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Lib/idlelib/pyshell.py

@@ -757,7 +757,7 @@ def runcommand(self, code):
     def runcode(self, code):
         "Override base class method"
         if self.tkconsole.executing:
-            self.interp.restart_subprocess()
+            self.restart_subprocess()
         self.checklinecache()
         debugger = self.debugger
         try: