[3.10] bpo-44273: Improve syntax error message for assigning to "..." (GH-26477) (GH-26478)

Use "ellipsis" instead of "Ellipsis" in syntax error messages to eliminate confusion with built-in variable Ellipsis.
(cherry picked from commit 39dd141)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>