[3.12] gh-111944: Add assignment expression parentheses requirements (GH-111977) (#112010)

Augment the list of places where parentheses are
required around assignnment statements.  In particular,
'a := 0' and 'a = b := 1' are syntax errors.
(cherry picked from commit 9a2f25d374f027f6509484d66e1c7bba03977b99)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>