Handle AST.expr in static object analysis

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

This fixes the remaining 4 failures under Python 3.9.

FAILED ropetest/advanced_oi_test.py::NewStaticOITest::test_static_oi_for_dicts_depending_on_append_function
FAILED ropetest/advanced_oi_test.py::NewStaticOITest::test_static_oi_for_dicts_depending_on_for_loops
FAILED ropetest/advanced_oi_test.py::NewStaticOITest::test_static_oi_for_dicts_depending_on_update
FAILED ropetest/advanced_oi_test.py::NewStaticOITest::test_static_oi_for_lists_per_object_for_set_item

Fixes: #299

rope/base/oi/soa.py

@@ -126,7 +126,7 @@ def _evaluate_assign_value(self, node, nodes, type_hint=False):
         for subscript, levels in nodes:
             instance = evaluate.eval_node(self.scope, subscript.value)
             args_pynames = [evaluate.eval_node(self.scope,
-                                               subscript.slice.value)]
+                                               subscript.slice)]
             value = rope.base.oi.soi._infer_assignment(
                 rope.base.pynames.AssignmentValue(node.value, levels,
                                                   type_hint=type_hint),
@@ -149,5 +149,5 @@ def __init__(self):
 
     def _added(self, node, levels):
         if isinstance(node, rope.base.ast.Subscript) and \
-           isinstance(node.slice, rope.base.ast.Index):
+           isinstance(node.slice, (rope.base.ast.Index, rope.base.ast.expr)):
             self.nodes.append((node, levels))