Handle AST.expr in _Subscript()

The ast module in Python 3.9 has some API changes. Quoting [1]:

    Simplified AST for subscription. Simple indices will be represented
    by their value, extended slices will be represented as tuples.
    Index(value) will return a value itself, ExtSlice(slices) will
    return Tuple(slices, Load()). (Contributed by Serhiy Storchaka in
    bpo-34822.)

[1] https://docs.python.org/3/whatsnew/3.9.html#changes-in-the-python-api

isinstance(thing, ast.Index) always return false in Python >= 3.9, so we
need to handle... whatever the value is now. ast.expr catches 20 of the
remaining 24 failures. The remaining 4 are resolved in the next patch.

Fixes: #299

rope/base/evaluate.py

@@ -307,6 +307,9 @@ def _Subscript(self, node):
         elif isinstance(node.slice, ast.Slice):
             self._call_function(node.value, '__getitem__',
                                 [node.slice])
+        elif isinstance(node.slice, ast.expr):
+            self._call_function(node.value, '__getitem__',
+                                [node.value])
 
     def _Slice(self, node):
         self.result = self._get_builtin_name('slice')