Always convert Text.index result to string.

This improves compatibility with Tcl 8.5, which would
otherwise return textindex objects.

Lib/lib-tk/Tkinter.py

@@ -2981,7 +2981,7 @@ class Text(Widget):
         return self.tk.call(self._w, "image", "names")
     def index(self, index):
         """Return the index in the form line.char for INDEX."""
-        return self.tk.call(self._w, 'index', index)
+        return str(self.tk.call(self._w, 'index', index))
     def insert(self, index, chars, *args):
         """Insert CHARS before the characters at INDEX. An additional
         tag can be given in ARGS. Additional CHARS and tags can follow in ARGS."""