[3.12] gh-88035: update doc-string of `epoch` in timemodule.c (GH-118076) (GH-118097)

Follow GH-88035, update doc-string of epoch in timemodule.c

The epoch is `January 1st, 1970 on all platforms`, according to
current documentation.
(cherry picked from commit 7c6cc00211772cc2afe0bc5e996b6d28f925d133)

Co-authored-by: lit <litlighilit@foxmail.com>