_sendfile_zerocopy leaks the file selector

[AZero13]

Bug report

Bug description:

_sendfile_zerocopy creates a fresh selector per call, registers the socket, and never uses it outside that scope.

We ought to close it.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-142403: Avoid leaking file descriptor in socket.py #142404

[picnixz]

I'm assuming this could have been introduced by #99907?