only do this sys.stderr replacing on CPython

Lib/lib2to3/pytree.py

@@ -743,7 +743,9 @@ class WildcardPattern(BasePattern):
         else:
             # The reason for this is that hitting the recursion limit usually
             # results in some ugly messages about how RuntimeErrors are being
-            # ignored.
+            # ignored. We only have to do this on CPython, though, because other
+            # implementations don't have this nasty bug in the first place.
+            if hasattr(sys, "getrefcount"):
                 save_stderr = sys.stderr
                 sys.stderr = StringIO()
             try:
@@ -759,6 +761,7 @@ class WildcardPattern(BasePattern):
                         r[self.name] = nodes[:count]
                     yield count, r
             finally:
+                if hasattr(sys, "getrefcount"):
                     sys.stderr = save_stderr
 
     def _iterative_matches(self, nodes):