[3.11] gh-104372: Drop the GIL around the vfork() call. (#104782) (#104958)

gh-104372: Drop the GIL around the vfork() call. (#104782)

On Linux where the `subprocess` module can use the `vfork` syscall for
faster spawning, prevent the parent process from blocking other threads
by dropping the GIL while it waits for the vfork'ed child process `exec`
outcome.  This prevents spawning a binary from a slow filesystem from
blocking the rest of the application.

Fixes #104372.

(cherry picked from commit d08679212d9af52dd074cd4a6abb440edb944c9c)