[2.7] bpo-35021: Fix assertion failures in _datetimemodule.c. (GH-10039) (GH-10617)

[2.7] bpo-35021: Fix assertion failures in _datetimemodule.c. (GH-10039) (GH-10617)

Fixes assertion failures in _datetimemodule.c
introduced in the previous fix (see bpo-31752).

Rather of trying to handle an int subclass as exact int,
let it to use overridden special methods, but check the
result of divmod().
(cherry picked from commit 3ec0f495163da3b7a15deb2805cec48aed432f58)

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