[3.12] gh-115809: Improve TimedRotatingFileHandler.getFilesToDelete() (GH-115812) (GH-116261)

Improve algorithm for computing which rolled-over log files to delete
in logging.TimedRotatingFileHandler. It is now reliable for handlers
without namer and with arbitrary deterministic namer that leaves
the datetime part in the file name unmodified.
(cherry picked from commit 87faec28c78f6fa8eaaebbd1ababf687c7508e71)

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