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.

Lib/test/test_signal.py

@@ -392,7 +392,6 @@ def handler(signum, frame):
         signal.signal(signum, handler)
 
         read, write = socket.socketpair()
-        read.setblocking(False)
         write.setblocking(False)
         signal.set_wakeup_fd(write.fileno())