Upgrade libuv to v1.46.0

[fantix]

• Fix subprocess/preexec_fn on macOS (darwin: Use posix_spawn to spawn subprocesses in macOS libuv/libuv#3064 changed to use posix_spawn) (CPython discussion bpo-35537)

(CPython code snippet of subprocess.py:)

            if (_USE_POSIX_SPAWN
                    and os.path.dirname(executable)
                    and preexec_fn is None
                    and not close_fds
                    and not pass_fds
                    and cwd is None
                    and (p2cread == -1 or p2cread > 2)
                    and (c2pwrite == -1 or c2pwrite > 2)
                    and (errwrite == -1 or errwrite > 2)
                    and not start_new_session
                    and process_group == -1
                    and gid is None
                    and gids is None
                    and uid is None
                    and umask < 0):
                self._posix_spawn(args, executable, env, restore_signals,
                                  p2cread, p2cwrite,
                                  c2pread, c2pwrite,
                                  errread, errwrite)
                return

[1st1]

Ideally we should make a PR to libuv to make it reliably configurable.