#1704474: mark refleak test as specific to CPython

Patch by Christian Hudon.

Lib/test/test_optparse.py

@@ -395,6 +395,7 @@ class TestOptionParser(BaseTest):
         self.assertRaises(self.parser.remove_option, ('foo',), None,
                           ValueError, "no such option 'foo'")
 
+    @support.impl_detail('Relies on sys.getrefcount', cpython=True)
     def test_refleak(self):
         # If an OptionParser is carrying around a reference to a large
         # object, various cycles can prevent it from being GC'd in