[3.13] gh-120683: Fix an error in logging.LogRecord timestamp (GH-120709) (GH-120933)

The integer part of the timestamp can be rounded up, while the millisecond
calculation truncates, causing the log timestamp to be wrong by up to 999 ms
(affected roughly 1 in 8 million timestamps).
(cherry picked from commit 1500a23f33f5a6d052ff1ef6383d9839928b8ff1)

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