bpo-38614: Use test.support.INTERNET_TIMEOUT constant (GH-17565)

Replace hardcoded timeout constants in tests with INTERNET_TIMEOUT of
test.support, so it's easier to ajdust this timeout for all tests at
once.
diff --git a/Lib/test/test_urllib2net.py b/Lib/test/test_urllib2net.py
index bb0500e..b3a5e89 100644
--- a/Lib/test/test_urllib2net.py
+++ b/Lib/test/test_urllib2net.py
@@ -10,8 +10,6 @@
 
 support.requires("network")
 
-TIMEOUT = 60  # seconds
-
 
 def _retry_thrice(func, exc, *args, **kwargs):
     for i in range(3):
@@ -227,7 +225,7 @@
 
                 with support.transient_internet(url):
                     try:
-                        f = urlopen(url, req, TIMEOUT)
+                        f = urlopen(url, req, support.INTERNET_TIMEOUT)
                     # urllib.error.URLError is a subclass of OSError
                     except OSError as err:
                         if expected_err: