Undefine the __str__ method

I haven't yet decided what a Simple Inkscape Scripting object should
look like as a string so for now it produces the same output as
repr(obj).

simpinkscr/simple_inkscape_scripting.py

@@ -113,6 +113,18 @@ def _python_to_svg_str(val):
         else:
             # Common case
             return ' '.join([_python_to_svg_str(v) for v in val])
+    try:
+        # Simple Inkscape Scripting objects are converted to a string of
+        # the form "url(#id)".
+        return val.get_inkex_object().get_id(as_url=2)
+    except AttributeError:
+        pass
+    try:
+        # inkex objects also are converted to a string of the form
+        # "url(#id)".
+        return val.get_id(as_url=2)
+    except AttributeError:
+        pass
     return str(val)  # Everything else is converted to a string as usual.
 
 
@@ -438,12 +450,6 @@ def __repr__(self):
              bbox.center,
              iobj.get_id(1))
 
-    def __str__(self):
-        '''Return the object as a string of the form "url(#id)".  This
-        enables the object to be used as a value in style key=value
-        arguments such as shape_inside.'''
-        return self._inkscape_obj.get_id(as_url=2)
-
     def __eq__(self, other):
         '''Two SimpleObjects are equal if they encapsulate the same
         inkex object.'''