bpo-34130: Fix test_signal.test_socket() (GH-8326)

bpo-34130: Fix test_signal.test_socket() (GH-8326)

test_signal.test_socket(): On Windows, sometimes even if the C signal handler
succeed to write the signal number into the write end of the socketpair, the
test fails with a BlockingIOError on the non-blocking read.recv(1) because the
read end of the socketpair didn't receive the byte yet.

Fix the race condition on Windows by setting the read end as blocking.