Improve error message in the parser when using un-parenthesised tuples in comprehensions

[pablogsal]

BPO	43017
Nosy	@aroberge, @lysnikolaou, @pablogsal
PRs	#24314

^{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 2021-01-31.22:53:06.078>
created_at = <Date 2021-01-24.18:00:46.555>
labels = []
title = 'Improve error message in the parser when using un-parenthesised tuples in comprehensions'
updated_at = <Date 2021-01-31.22:53:06.078>
user = 'https://github.com/pablogsal'

bugs.python.org fields:

activity = <Date 2021-01-31.22:53:06.078>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2021-01-31.22:53:06.078>
closer = 'pablogsal'
components = []
creation = <Date 2021-01-24.18:00:46.555>
creator = 'pablogsal'
dependencies = []
files = []
hgrepos = []
issue_num = 43017
keywords = ['patch']
message_count = 3.0
messages = ['385586', '385655', '386045']
nosy_count = 3.0
nosy_names = ['aroberge', 'lys.nikolaou', 'pablogsal']
pr_nums = ['24314']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue43017'
versions = []

[pablogsal]

I have seen this confusion many users over the years. Instead of showing:

❯ python -c '[x,z for x,y in something]'
File "<string>", line 1
[x,z for x,y in something]
^
SyntaxError: invalid syntax

We can show:

❯ ./python -c '[x,z for x,y in something]'
File "<string>", line 1
[x,z for x,y in something]
^
SyntaxError: did you forgot parentheses around the comprehension target?

[aroberge]

Such a change would be very useful and appreciated by other users as reported on aroberge/friendly#167 and asked about on StackOverflow
https://stackoverflow.com/questions/60986850/why-does-creating-a-list-of-tuples-using-list-comprehension-requires-parentheses

[pablogsal]

New changeset 835f14f by Pablo Galindo in branch 'master':
bpo-43017: Improve error message for unparenthesised tuples in comprehensions (GH24314)
835f14f