bpo-32517: fix test_read_pty_output() hangs on macOS 10.13.2+ (GH-6037) (GH-6113)

test_asyncio hangs indefinitely on macOS 10.13.2+ on `read_pty_output()`
using the KqueueSelector. Closing `proto.transport` (as is done in
`write_pty_output()`) seems to fix it.
(cherry picked from commit 12f74d8608c15cacd9d5786524e2be9ca36f007e)

Co-authored-by: Nathan Henrie <n8henrie@users.noreply.github.com>

Also, re-enable test_read_pty_output on macOS.