Improve syntax errors for missing commas

[pablogsal]

BPO	43822
Nosy	@pablogsal, @miss-islington
PRs	bpo-43822: Improve syntax errors for missing commas #25377 bpo-43822: When reporting parser custom errors, check first for tokenizer errors #25866 [3.10] bpo-43822: Prioritize tokenizer errors over custom syntax errors when raising parser exceptions (GH-25866) #25869 [3.10] bpo-43822: Prioritize tokenizer errors over custom syntax errors when raising parser exceptions (GH-25866) #25870

^{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 2021-04-15.22:07:56.465>
created_at = <Date 2021-04-13.01:30:43.793>
labels = []
title = 'Improve syntax errors for missing commas'
updated_at = <Date 2021-07-18.15:31:01.858>
user = 'https://github.com/pablogsal'

bugs.python.org fields:

activity = <Date 2021-07-18.15:31:01.858>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2021-04-15.22:07:56.465>
closer = 'pablogsal'
components = []
creation = <Date 2021-04-13.01:30:43.793>
creator = 'pablogsal'
dependencies = []
files = []
hgrepos = []
issue_num = 43822
keywords = ['patch']
message_count = 4.0
messages = ['390916', '391152', '392846', '392847']
nosy_count = 2.0
nosy_names = ['pablogsal', 'miss-islington']
pr_nums = ['25377', '25866', '25869', '25870']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue43822'
versions = []

[pablogsal]

Given that is quite common to forgot a comma in containers, function calls, ...etc and that the current error is:

>>> [a, b, c d]
  File "<stdin>", line 1
    [a, b, c d]
             ^
SyntaxError: invalid syntax

We could improve the user experience quite a lot if we mention that probably the user forgot a comma:

>>> [a, b, c d]
  File "<stdin>", line 1
    [a, b, c d]
             ^
SyntaxError: invalid syntax. Perhaps you forgot a comma?

[pablogsal]

New changeset b280248 by Pablo Galindo in branch 'master':
bpo-43822: Improve syntax errors for missing commas (GH-25377)
b280248

[miss-islington]

New changeset 756b7b9 by Miss Islington (bot) in branch '3.10':
bpo-43822: Prioritize tokenizer errors over custom syntax errors when raising parser exceptions (GH-25866)
756b7b9

[miss-islington]

New changeset d1ff838 by Miss Islington (bot) in branch '3.10':
bpo-43822: Prioritize tokenizer errors over custom syntax errors when raising parser exceptions (GH-25866)
d1ff838