bpo-41471: Ignore invalid prefix lengths in system proxy settings on macOS (GH-22762)

diff --git a/Lib/test/test_urllib2.py b/Lib/test/test_urllib2.py
index f202f97..12ad6ae 100644
--- a/Lib/test/test_urllib2.py
+++ b/Lib/test/test_urllib2.py
@@ -1449,6 +1449,18 @@
         bypass = {'exclude_simple': True, 'exceptions': []}
         self.assertTrue(_proxy_bypass_macosx_sysconf('test', bypass))
 
+        # Check that invalid prefix lengths are ignored
+        bypass = {
+            'exclude_simple': False,
+            'exceptions': [ '10.0.0.0/40', '172.19.10.0/24' ]
+        }
+        host = '172.19.10.5'
+        self.assertTrue(_proxy_bypass_macosx_sysconf(host, bypass),
+                        'expected bypass of %s to be True' % host)
+        host = '10.0.1.5'
+        self.assertFalse(_proxy_bypass_macosx_sysconf(host, bypass),
+                        'expected bypass of %s to be False' % host)
+
     def check_basic_auth(self, headers, realm):
         with self.subTest(realm=realm, headers=headers):
             opener = OpenerDirector()