Allow import of multiprocessing.connection even if _multiprocessing is missing

[hoodmane]

Bug report

Bug description:

Currently if _multiprocessing is missing then from multiprocessing import connection raises an ImportError. It would be nice to delay the error until someone attempts to actually use multiprocessing.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-114807: multiprocessing: don't raise ImportError if _multiprocessing is missing #114808