[3.9] bpo-42381: Allow walrus in set literals and set comprehensions (GH-23332) ...

[3.9] bpo-42381: Allow walrus in set literals and set comprehensions (GH-23332) (GH-23333)

Currently walruses are not allowerd in set literals and set comprehensions:

>>> {y := 4, 4**2, 3**3}
  File "<stdin>", line 1
    {y := 4, 4**2, 3**3}
       ^
SyntaxError: invalid syntax

but they should be allowed as well per PEP 572.
(cherry picked from commit b0aba1fcdc3da952698d99aec2334faa79a8b68c)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>