Python -builtin __contains__ fix for map and set like containers.

Fix when using -builtin and wrapping std::map, std::set, std::unordered_map or
std::unordered_set to ensure __contains__ is called. This is a wrapper for the STL
container's find method. Without it, Python will do its own slower sequence search.

Lib/python/std_set.i

@@ -36,6 +36,10 @@
   %swig_sequence_iterator(set);
   %swig_container_methods(set);
 
+#if defined(SWIGPYTHON_BUILTIN)
+  %feature("python:slot", "sq_contains", functype="objobjproc") __contains__;
+#endif
+
   %extend  {
      void append(value_type x) {
        self->insert(x);