bpo-28240: timeit: Update repeat() doc (GH-7419) (GH-7457)

Document that the default value of repeat changed from 3 to 5 in
Python 3.7.
(cherry picked from commit 3ef769fcd378a7f1cda19c0dfec2e79613d79e48)

Co-authored-by: Victor Stinner <vstinner@redhat.com>
2 files changed