gh-149879: Fix test_asyncio on Cygwin

[vstinner]

Skip tests on UNIX sockets since they hang on Cygwin.

• Issue: Enhance Cygwin support #149879

[vstinner]

The fix is incomplete, the following tests still fail:

test_asyncio.test_events
    test_ssl_connect_accepted_socket()
    test_subprocess_kill()
    test_subprocess_terminate()
    test_subprocess_send_signal()
    test_subprocess_stderr()
    test_subprocess_stderr_redirect_to_stdout()

test_asyncio.test_ssl
    test_remote_shutdown_receives_trailing_data_on_slow_socket()

test_asyncio.test_subprocess
    test_cancel_post_init()

[vstinner]

I found a fix for subprocess tests (use a socketpair for stdin).

test_remote_shutdown_receives_trailing_data_on_slow_socket() stil hangs. It can be fixed later.