only do this sys.stderr replacing on CPython

Lib/lib2to3/pytree.py

@@ -743,9 +743,11 @@ def generate_matches(self, nodes):
         else:
             # The reason for this is that hitting the recursion limit usually
             # results in some ugly messages about how RuntimeErrors are being
-            # ignored.
-            save_stderr = sys.stderr
-            sys.stderr = StringIO()
+            # ignored. We don't do this on non-CPython implementation because
+            # they don't have this problem.
+            if hasattr(sys, "getrefcount"):
+                save_stderr = sys.stderr
+                sys.stderr = StringIO()
             try:
                 for count, r in self._recursive_matches(nodes, 0):
                     if self.name:
@@ -759,7 +761,8 @@ def generate_matches(self, nodes):
                         r[self.name] = nodes[:count]
                     yield count, r
             finally:
-                sys.stderr = save_stderr
+                if hasattr(sys, "getrefcount"):
+                    sys.stderr = save_stderr
 
     def _iterative_matches(self, nodes):
         """Helper to iteratively yield the matches."""