bpo-42381: Allow walrus in set literals and set comprehensions (GH-23332)

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