bpo-37421: urllib.request tests call urlcleanup() (GH-14529)

urllib.request tests now call urlcleanup() to remove temporary files
created by urlretrieve() tests and to clear the _opener global
variable set by urlopen() and functions calling indirectly urlopen().

regrtest now checks if urllib.request._url_tempfiles and
urllib.request._opener are changed by tests.
diff --git a/Lib/test/test_urllib2net.py b/Lib/test/test_urllib2net.py
index 0f43d71..040a2ce 100644
--- a/Lib/test/test_urllib2net.py
+++ b/Lib/test/test_urllib2net.py
@@ -82,6 +82,9 @@
 class CloseSocketTest(unittest.TestCase):
 
     def test_close(self):
+        # clear _opener global variable
+        self.addCleanup(urllib.request.urlcleanup)
+
         # calling .close() on urllib2's response objects should close the
         # underlying socket
         url = support.TEST_HTTP_URL
@@ -257,6 +260,10 @@
 
 
 class TimeoutTest(unittest.TestCase):
+    def setUp(self):
+        # clear _opener global variable
+        self.addCleanup(urllib.request.urlcleanup)
+
     def test_http_basic(self):
         self.assertIsNone(socket.getdefaulttimeout())
         url = support.TEST_HTTP_URL