Improve syntax error message for assigning to "..."

[serhiy-storchaka]

BPO	44278
Nosy	@serhiy-storchaka, @pablogsal, @isidentical
PRs	bpo-44273: Improve syntax error message for assigning to "..." #26477
Superseder	bpo-44273: Assigning to Ellipsis should be the same as assigning to debug

^{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 2021-06-01.10:43:16.587>
created_at = <Date 2021-06-01.10:36:54.917>
labels = ['interpreter-core', 'type-feature', '3.10', '3.11']
title = 'Improve syntax error message for assigning to "..."'
updated_at = <Date 2021-06-01.10:43:16.587>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2021-06-01.10:43:16.587>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2021-06-01.10:43:16.587>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2021-06-01.10:36:54.917>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 44278
keywords = ['patch']
message_count = 2.0
messages = ['394849', '394851']
nosy_count = 3.0
nosy_names = ['serhiy.storchaka', 'pablogsal', 'BTaskaya']
pr_nums = ['26477']
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '44273'
type = 'enhancement'
url = 'https://bugs.python.org/issue44278'
versions = ['Python 3.10', 'Python 3.11']

[serhiy-storchaka]

Some users can be confused by syntax error message for assigning to ellipsis [1]:

>>> ... = 1
  File "<stdin>", line 1
    ... = 1
    ^^^
SyntaxError: cannot assign to Ellipsis here. Maybe you meant '==' instead of '='?

because Ellipsis is a name of a builtin variable Ellipsis, and it is possible to assign to variable named Ellipsis.

>>> Ellipsis = 1
>>> Ellipsis
1

I think that using lowercase spelling of "ellipsis" will eliminate confusion.

[1] https://mail.python.org/archives/list/python-ideas@python.org/thread/KDQ3OHALLXVZJIGPC4BMPVS2XH3VFPJV/

[isidentical]

I think this is a duplicate of bpo-44273.