bpo-40275: Move TransientResource to test_urllib2net (GH-20812)

Move TransientResource, time_out, socket_peer_reset and
ioerror_peer_reset from test.support to test_urllib2net.

Remove "import errno" from test.support.

Lib/test/test_urllib2net.py

@@ -1,3 +1,4 @@
+import errno
 import unittest
 from test import support
 from test.support import socket_helper
@@ -39,6 +40,39 @@ _urlopen_with_retry = _wrap_with_retry_thrice(urllib.request.urlopen,
                                               urllib.error.URLError)
 
 
+class TransientResource(object):
+
+    """Raise ResourceDenied if an exception is raised while the context manager
+    is in effect that matches the specified exception and attributes."""
+
+    def __init__(self, exc, **kwargs):
+        self.exc = exc
+        self.attrs = kwargs
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, type_=None, value=None, traceback=None):
+        """If type_ is a subclass of self.exc and value has attributes matching
+        self.attrs, raise ResourceDenied.  Otherwise let the exception
+        propagate (if any)."""
+        if type_ is not None and issubclass(self.exc, type_):
+            for attr, attr_value in self.attrs.items():
+                if not hasattr(value, attr):
+                    break
+                if getattr(value, attr) != attr_value:
+                    break
+            else:
+                raise ResourceDenied("an optional resource is not available")
+
+# Context managers that raise ResourceDenied when various issues
+# with the Internet connection manifest themselves as exceptions.
+# XXX deprecate these and use transient_internet() instead
+time_out = TransientResource(OSError, errno=errno.ETIMEDOUT)
+socket_peer_reset = TransientResource(OSError, errno=errno.ECONNRESET)
+ioerror_peer_reset = TransientResource(OSError, errno=errno.ECONNRESET)
+
+
 class AuthTests(unittest.TestCase):
     """Tests urllib2 authentication features."""
 
@@ -237,9 +271,9 @@ class OtherNetworkTests(unittest.TestCase):
                             raise
                     else:
                         try:
-                            with support.time_out, \
-                                 support.socket_peer_reset, \
-                                 support.ioerror_peer_reset:
+                            with time_out, \
+                                 socket_peer_reset, \
+                                 ioerror_peer_reset:
                                 buf = f.read()
                                 debug("read %d bytes" % len(buf))
                         except socket.timeout: