Fix error "'_ExpressionVisitor' object has no attribute 'defineds'"

[lieryan]

Description

In rope master, code that uses inline assignment (:= operator), doing a AutoImport.generate_cache() produces an error:

Traceback (most recent call last):
  File "ref.py", line 8, in <module>
    importer.generate_cache()
  File "/home/user/rope/rope/contrib/autoimport.py", line 107, in generate_cache
    self.update_resource(file, underlined)
  File "/home/user/rope/rope/contrib/autoimport.py", line 160, in update_resource
    self._add_names(pymodule, modname, underlined)
  File "/home/user/rope/rope/contrib/autoimport.py", line 183, in _add_names
    attributes = pymodule._get_structural_attributes()
  File "/home/user/rope/rope/base/utils/__init__.py", line 32, in newfunc
    return func(self, *args, **kwds)
  File "/home/user/rope/rope/base/pyobjects.py", line 179, in _get_structural_attributes
    self.structural_attributes = self._create_structural_attributes()
  File "/home/user/rope/rope/base/pyobjects.py", line 232, in _create_structural_attributes
    ast.walk(child, new_visitor)
  File "/home/user/rope/rope/base/ast.py", line 40, in walk
    return method(node)
  File "/home/user/rope/rope/base/pyobjectsdef.py", line 460, in _Assign
    ast.walk(node, _AssignVisitor(self))
  File "/home/user/rope/rope/base/ast.py", line 40, in walk
    return method(node)
  File "/home/user/rope/rope/base/pyobjectsdef.py", line 380, in _Assign
    ast.walk(node.value, _ExpressionVisitor(self.scope_visitor))
  File "/home/user/rope/rope/base/ast.py", line 42, in walk
    walk(child, walker)
  File "/home/user/rope/rope/base/ast.py", line 40, in walk
    return method(node)
  File "/home/user/rope/rope/base/pyobjectsdef.py", line 367, in _NamedExpr
    ast.walk(node.target, _AssignVisitor(self))
  File "/home/user/rope/rope/base/ast.py", line 40, in walk
    return method(node)
  File "/home/user/rope/rope/base/pyobjectsdef.py", line 392, in _Name
    self._assigned(node.id, assignment)
  File "/home/user/rope/rope/base/pyobjectsdef.py", line 383, in _assigned
    print(self.scope_visitor, self.scope_visitor.defineds)
AttributeError: '_ExpressionVisitor' object has no attribute 'defineds'

Checklist (delete if not relevant):

• I have added tests that prove my fix is effective or that my feature works