Issue #18963: skip test_selectors.test_above_fd_setsize on older OS X versions.

Lib/test/test_selectors.py

@@ -301,6 +301,8 @@ class BaseSelectorTestCase(unittest.TestCase):
 
 class ScalableSelectorMixIn:
 
+    # see issue #18963 for why it's skipped on older OS X versions
+    @support.requires_mac_ver(10, 5)
     @unittest.skipUnless(resource, "Test needs resource module")
     def test_above_fd_setsize(self):
         # A scalable implementation should have no problem with more than