bpo-32270: Don't close stdin/out/err in pass_fds (GH-6242)

bpo-32270: Don't close stdin/out/err in pass_fds (GH-6242)

When subprocess.Popen() stdin= stdout= or stderr= handles are specified
and appear in pass_fds=, don't close the original fds after dup'ing them.

This implementation and unittest primarily came from @izbyshev (see the PR)

See also https://github.com/izbyshev/cpython/commit/b89b52f28490b69142d5c061604b3a3989cec66c

This also removes the old manual p2cread, c2pwrite, and errwrite closing logic
as inheritable flags and _close_open_fds takes care of that properly today without special treatment.

This code is within child_exec() where it is the only thread so there is no
race condition between the dup and _Py_set_inheritable_async_safe call.