gh-95027: Fix regrtest stdout encoding on Windows (GH-98492)

On Windows, when the Python test suite is run with the -jN option,
the ANSI code page is now used as the encoding for the stdout
temporary file, rather than using UTF-8 which can lead to decoding
errors.
(cherry picked from commit ec1f6f5f)

Co-authored-by: Victor Stinner <vstinner@python.org>