[3.13] gh-137754: Fix import of zoneinfo if _datetime is not available (GH-137845) (GH-138085)

Both modules should use the Python implementation in that case.
(cherry picked from commit 6620ef0ff62dea2632bedf9afa88aa4780a2c9d9)

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