Speccial case the unused '_' in warnings.

Cython/Compiler/FlowControl.py

@@ -616,7 +616,8 @@ def check_definitions(flow, compiler_directives):
     # Unused entries
     for entry in flow.entries:
         if (not entry.cf_references and not entry.is_pyclass_attr
-            and not entry.in_closure):
+            and not entry.in_closure
+            and entry.name != '_'):
             if entry.is_arg:
                 if warn_unused_arg:
                     messages.warning(entry.pos, "Unused argument '%s'" %