bpo-39763: distutils.spawn now uses subprocess (GH-18743)

Reimplement distutils.spawn.spawn() function with the subprocess
module.

setup.py now uses a basic implementation of the subprocess module if
the subprocess module is not available: before required C extension
modules are built.

Lib/distutils/tests/test_spawn.py

@@ -8,7 +8,6 @@ from test.support import run_unittest, unix_shell
 from test import support as test_support
 
 from distutils.spawn import find_executable
-from distutils.spawn import _nt_quote_args
 from distutils.spawn import spawn
 from distutils.errors import DistutilsExecError
 from distutils.tests import support
@@ -17,16 +16,6 @@ class SpawnTestCase(support.TempdirManager,
                     support.LoggingSilencer,
                     unittest.TestCase):
 
-    def test_nt_quote_args(self):
-
-        for (args, wanted) in ((['with space', 'nospace'],
-                                ['"with space"', 'nospace']),
-                               (['nochange', 'nospace'],
-                                ['nochange', 'nospace'])):
-            res = _nt_quote_args(args)
-            self.assertEqual(res, wanted)
-
-
     @unittest.skipUnless(os.name in ('nt', 'posix'),
                          'Runs only under posix or nt')
     def test_spawn(self):