gh-82836: fix private network check (GH-97733)

Fixes private checks for network objects. The previous method would incorrectly return True for a private check in cases such as "0.0.0.0/0".
(cherry picked from commit ed391090cc)

Co-authored-by: Pete Wicken <2273100+JamoBox@users.noreply.github.com>

Lib/ipaddress.py

@@ -1077,15 +1077,16 @@ class _BaseNetwork(_IPAddressBase):
 
     @property
     def is_private(self):
-        """Test if this address is allocated for private networks.
+        """Test if this network belongs to a private range.
 
         Returns:
-            A boolean, True if the address is reserved per
+            A boolean, True if the network is reserved per
             iana-ipv4-special-registry or iana-ipv6-special-registry.
 
         """
-        return (self.network_address.is_private and
-                self.broadcast_address.is_private)
+        return any(self.network_address in priv_network and
+                   self.broadcast_address in priv_network
+                   for priv_network in self._constants._private_networks)
 
     @property
     def is_global(self):
@@ -1122,6 +1123,15 @@ class _BaseNetwork(_IPAddressBase):
         return (self.network_address.is_loopback and
                 self.broadcast_address.is_loopback)
 
+
+class _BaseConstants:
+
+    _private_networks = []
+
+
+_BaseNetwork._constants = _BaseConstants
+
+
 class _BaseV4:
 
     """Base IPv4 object.
@@ -1561,6 +1571,7 @@ class _IPv4Constants:
 
 
 IPv4Address._constants = _IPv4Constants
+IPv4Network._constants = _IPv4Constants
 
 
 class _BaseV6:
@@ -2285,3 +2296,4 @@ class _IPv6Constants:
 
 
 IPv6Address._constants = _IPv6Constants
+IPv6Network._constants = _IPv6Constants