bpo-34130: Fix test_signal.test_socket() (GH-8326) (GH-8330)

bpo-34130: Fix test_signal.test_socket() (GH-8326) (GH-8330)

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.
(cherry picked from commit 99bb6df66a42625367c4f38e6802c8bb527baf4a)

Co-authored-by: Victor Stinner <vstinner@redhat.com>