[3.14] gh-75572: Speed up test_xpickle (GH-144393) (GH-144403)

Run a long living subprocess which handles multiple requests instead of
running a new subprocess for each request.
(cherry picked from commit 29acc08c8dad664cd5713cb392e5beba65724c10)

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