gh-146260: Improve syntax error for assignment expression inside conditional expression

[brianschubert]

Demo:

>>> (
...     1
...     if x := True
...     else 2
... )
... 
  File "<python-input-0>", line 3
    if x := True
       ^^^^^^^^^
SyntaxError: assignment expression must be parenthesized inside conditional expression

>>> (
...     1
...     if (x := True)
...     else 2
... )
1

• Issue: Incorrect SyntaxError location for walrus operator in multi-line conditional expression #146260

[brianschubert]

See Guido's comment in the issue about relaxing the syntax instead: #146260 (comment)