Python builtin C++11 STL fixes

Compilation fixes for std::multimap, std::unordered_map and
std::unordered_multimap.

Lib/python/std_unordered_map.i

@@ -226,7 +226,17 @@
 
 %define %swig_unordered_map_methods(Map...)
   %swig_unordered_map_common(Map)
+
+#if defined(SWIGPYTHON_BUILTIN)
+  %feature("python:slot", "mp_ass_subscript", functype="objobjargproc") __setitem__;
+#endif
+
   %extend {
+    // This will be called through the mp_ass_subscript slot to delete an entry.
+    void __setitem__(const key_type& key) {
+      self->erase(key);
+    }
+
     void __setitem__(const key_type& key, const mapped_type& x) throw (std::out_of_range) {
       (*self)[key] = x;
     }