bpo-46503: Prevent an assert from firing when parsing some invalid \N sequences in f-strings. (GH-30865) (GH-30866)

* bpo-46503: Prevent an assert from firing.  Also fix one nearby tiny PEP-7 nit.

* Added blurb.
(cherry picked from commit 0daf72194bd4e31de7f12020685bb39a14d6f45e)

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

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