The _posixsubprocess module is now required on POSIX.

Remove the pure Python POSIX subprocess implementation.

If non-CPython VMs (are there any for 3.x yet?) were somehow depending
on this, they already have the exact same set of problems with Python
code being executed after os.fork() that _posixsubprocess was written
to deal with.  They should implement an equivalent outside of Python.

Lib/test/test_subprocess.py

@@ -1495,28 +1495,6 @@ class ProcessTestCaseNoPoll(ProcessTestCase):
         ProcessTestCase.tearDown(self)
 
 
-@unittest.skipUnless(getattr(subprocess, '_posixsubprocess', False),
-                     "_posixsubprocess extension module not found.")
-class ProcessTestCasePOSIXPurePython(ProcessTestCase, POSIXProcessTestCase):
-    @classmethod
-    def setUpClass(cls):
-        global subprocess
-        assert subprocess._posixsubprocess
-        # Reimport subprocess while forcing _posixsubprocess to not exist.
-        with support.check_warnings(('.*_posixsubprocess .* not being used.*',
-                                     RuntimeWarning)):
-            subprocess = support.import_fresh_module(
-                    'subprocess', blocked=['_posixsubprocess'])
-        assert not subprocess._posixsubprocess
-
-    @classmethod
-    def tearDownClass(cls):
-        global subprocess
-        # Reimport subprocess as it should be, restoring order to the universe.
-        subprocess = support.import_fresh_module('subprocess')
-        assert subprocess._posixsubprocess
-
-
 class HelperFunctionTests(unittest.TestCase):
     @unittest.skipIf(mswindows, "errno and EINTR make no sense on windows")
     def test_eintr_retry_call(self):