bpo-35537: subprocess uses os.posix_spawn in some cases (GH-11452)

The subprocess module can now use the os.posix_spawn() function
in some cases for better performance. Currently, it is only used on macOS
and Linux (using glibc 2.24 or newer) if all these conditions are met:

* executable path contains a directory
* close_fds=False
* preexec_fn, pass_fds, cwd, stdin, stdout, stderr
  and start_new_session parameters are not set

Co-authored-by: Joannah Nanjekye <nanjekyejoannah@gmail.com>