test_sys globally requires _datetime for cpython_only test

[youknowone]

Bug report

Bug description:

_datetime is only being used by @cpython_only test, but imported on top level

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-143959: Split datetime tests requiring _datetime #143997
• gh-143959 make test_sys import _datetime inside the test #144003