Documentation

The documentation of os.cpu_count() mentions that to get the the number of usable CPUs one can use len(os.sched_getaffinity(0)). However that's not available in all platforms, for instance in macOS.
For Unix platforms a better approach would be to use os.sysconf('SC_NPROCESSORS_ONLN'), which is also not fully standard, however at least it has been proposed for the next POSIX standard (https://www.austingroupbugs.net/view.php?id=339) and is currently available in many UNIX platforms.