[3.10] gh-90622: Do not spawn ProcessPool workers on demand via fork method. (GH-91598) (GH-92497) (#92499)

Do not spawn ProcessPool workers on demand when they spawn via fork.

This avoids potential deadlocks in the child processes due to forking from
a multithreaded process..
(cherry picked from commit ebb37fc3)

Co-authored-by: Gregory P. Smith <greg@krypto.org>
(cherry picked from commit b795376a)

Co-authored-by: Gregory P. Smith <greg@krypto.org>

Co-authored-by: Gregory P. Smith <greg@krypto.org>