Improve error messages regarding unclosed parentheses

[pablogsal]

BPO	42864
Nosy	@lysnikolaou, @pablogsal, @miss-islington
PRs	#24161 #24266 #24273

^{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 2021-01-20.00:07:04.832>
created_at = <Date 2021-01-08.03:03:30.904>
labels = ['3.10']
title = 'Improve error messages regarding unclosed parentheses'
updated_at = <Date 2021-01-20.19:12:05.406>
user = 'https://github.com/pablogsal'

bugs.python.org fields:

activity = <Date 2021-01-20.19:12:05.406>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2021-01-20.00:07:04.832>
closer = 'pablogsal'
components = []
creation = <Date 2021-01-08.03:03:30.904>
creator = 'pablogsal'
dependencies = []
files = []
hgrepos = []
issue_num = 42864
keywords = ['patch']
message_count = 4.0
messages = ['384630', '385307', '385338', '385362']
nosy_count = 3.0
nosy_names = ['lys.nikolaou', 'pablogsal', 'miss-islington']
pr_nums = ['24161', '24266', '24273']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue42864'
versions = ['Python 3.10']

[pablogsal]

Consider this file:

x = (

The error that we get is:

❯ python ../a.py
File "/home/pablogsal/github/python/master/../a.py", line 6

^

SyntaxError: unexpected EOF while parsin

This is quite uninformative of the actual problem, which is the closed parentheses.

The same happens with something like this:

( 1+2

File "/home/pablogsal/github/python/master/lel.py", line 3

^

SyntaxError: unexpected EOF while parsing

With some effort, we can include the location of the unclosed parentheses.

[pablogsal]

New changeset d6d6371 by Pablo Galindo in branch 'master':
bpo-42864: Improve error messages regarding unclosed parentheses (GH-24161)
d6d6371

[pablogsal]

New changeset ae7d3cd by Pablo Galindo in branch 'master':
bpo-42864: Fix compiler warning in the tokenizer with the new paren stack for column numbers (GH-24266)
ae7d3cd

[miss-islington]

New changeset c3f167d by Pablo Galindo in branch 'master':
bpo-42864: Simplify the tokenizer exceptions after generic SyntaxError (GH-24273)
c3f167d