Improve syntax errors for invalid if expressions

[miguendes]

BPO	44792
Nosy	@aroberge, @pablogsal, @miss-islington, @miguendes
PRs	bpo-44792: Improve syntax errors for if expressions #27506 [3.10] bpo-44792: Improve syntax errors for if expressions (GH-27506) #27565

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-08-02.19:05:42.925>
created_at = <Date 2021-07-31.11:25:20.977>
labels = []
title = 'Improve syntax errors for invalid if expressions'
updated_at = <Date 2021-08-02.19:05:43.048>
user = 'https://github.com/miguendes'

bugs.python.org fields:

activity = <Date 2021-08-02.19:05:43.048>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2021-08-02.19:05:42.925>
closer = 'pablogsal'
components = []
creation = <Date 2021-07-31.11:25:20.977>
creator = 'miguendes'
dependencies = []
files = []
hgrepos = []
issue_num = 44792
keywords = ['patch']
message_count = 3.0
messages = ['398633', '398787', '398799']
nosy_count = 4.0
nosy_names = ['aroberge', 'pablogsal', 'miss-islington', 'miguendes']
pr_nums = ['27506', '27565']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue44792'
versions = []

[miguendes]

Hi, I was playing around with Python's grammar and noticed that the error message for if expression is generic, so not very informative.

I decided to improve it slightly.

*From*:

>>> a = 42 if True
  File "<stdin>", line 1
    a = 42 if True
                  ^
SyntaxError: invalid syntax

*To*:

$ ./python
Python 3.10.0b4 (tags/v3.10.0b4-dirty:2ba4b20854, Jul 31 2021, 11:50:15) [GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 42 if True
  File "<stdin>", line 1
    a = 42 if True
                  ^
SyntaxError: invalid syntax. Conditional expression expected an 'else' here.

[pablogsal]

New changeset 28b6dc9 by Miguel Brito in branch 'main':
bpo-44792: Improve syntax errors for if expressions (GH-27506)
28b6dc9

[pablogsal]

New changeset 5671762 by Miss Islington (bot) in branch '3.10':
bpo-44792: Improve syntax errors for if expressions (GH-27506) (GH-27565)
5671762