[3.6] bpo-33329: Fix multiprocessing regression on newer glibcs (GH-6575) (GH-6582)

[3.6] bpo-33329: Fix multiprocessing regression on newer glibcs (GH-6575) (GH-6582)

Starting with glibc 2.27.9000-xxx, sigaddset() can return EINVAL for some
reserved signal numbers between 1 and NSIG.  The `range(1, NSIG)` idiom
is commonly used to select all signals for blocking with `pthread_sigmask`.
So we ignore the sigaddset() return value until we expose sigfillset()
to provide a better idiom.
(cherry picked from commit 25038ecfb665bef641abf8cb61afff7505b0e008)