bpo-36019: Use pythontest.net instead of example.com in network tests (GH-11941)

(cherry picked from commit a40681dd5db8deaf05a635eecb91498dac882aa4)

Co-authored-by: Stéphane Wirtel <stephane@wirtel.be>
diff --git a/Lib/test/test_urllib.py b/Lib/test/test_urllib.py
index c292d74..2ac73b5 100644
--- a/Lib/test/test_urllib.py
+++ b/Lib/test/test_urllib.py
@@ -712,7 +712,7 @@
 
         with self.assertRaises(urllib.error.ContentTooShortError):
             try:
-                urllib.request.urlretrieve('http://example.com/',
+                urllib.request.urlretrieve(support.TEST_HTTP_URL,
                                            reporthook=_reporthook)
             finally:
                 self.unfakehttp()
@@ -729,7 +729,7 @@
 ''')
         with self.assertRaises(urllib.error.ContentTooShortError):
             try:
-                urllib.request.urlretrieve('http://example.com/')
+                urllib.request.urlretrieve(support.TEST_HTTP_URL)
             finally:
                 self.unfakehttp()