[3.14] gh-134857: Improve error report for doctests run with unittest (GH-134858) (GH-134903)

Remove doctest module frames from tracebacks and redundant newline
character from a failure message.
(cherry picked from commit cb8a72b301f47e76d93a7fe5b259e9a5758792e1)

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