bpo-46762: Fix an assert failure in f-strings where > or < is the last character if the f-string is missing a trailing right brace. (GH-31365)

(cherry picked from commit ffd9f8ff84ed53c956b16d027f7d2926ea631051)

Co-authored-by: Eric V. Smith <ericvsmith@users.noreply.github.com>