bpo-37421: Fix test_distutils.test_build_ext() (GH-14564)

test_distutils.test_build_ext() is now able to remove the temporary
directory on Windows: don't import the newly built C extension ("xx")
in the current process, but test it in a separated process.

Lib/distutils/tests/support.py

@@ -6,6 +6,7 @@ import tempfile
 import unittest
 import sysconfig
 from copy import deepcopy
+import test.support
 
 from distutils import log
 from distutils.log import DEBUG, INFO, WARN, ERROR, FATAL
@@ -64,8 +65,8 @@ class TempdirManager(object):
         os.chdir(self.old_cwd)
         super().tearDown()
         while self.tempdirs:
-            d = self.tempdirs.pop()
-            shutil.rmtree(d, os.name in ('nt', 'cygwin'))
+            tmpdir = self.tempdirs.pop()
+            test.support.rmtree(tmpdir)
 
     def mkdtemp(self):
         """Create a temporary directory that will be cleaned up.