[3.13] gh-117398: Add multiphase support to _datetime (gh-119694)

[3.13] gh-117398: Add multiphase support to _datetime (gh-119694)

This is an unrevert of d58ebf0 (gh-119636), which was reverted by 9216a53 (gh-119639) due to problems which have been resolved.

This is minimal support for multiphase init. Subinterpreters are not supported yet. That will be addressed in a later change.

(cherry picked from commit 3e8b609)

Co-authored-by: Erlend E. Aasland erlend@python.org