bpo-33608: Factor out a private, per-interpreter _Py_AddPendingCall(). (gh-12360)

This is effectively an un-revert of #11617 and #12024 (reverted in #12159). Portions of those were merged in other PRs (with lower risk) and this represents the remainder. Note that I found 3 different bugs in the original PRs and have fixed them here.