Fix dependency of SwigPySequence_Base fragment

This fragment needs to depend on StdTraits fragment as it specializes the
template struct traits<> defined there and it could happen that the struct was
not declared before being specialized, resulting in compilation errors in the
generated code.

It doesn't seem to need to depend on stddef.h inclusion, however, so replace
it with the correct dependency instead of keeping both.

Lib/python/pycontainer.swg

@@ -102,7 +102,7 @@ namespace swig {
 } 
 }
 
-%fragment("SwigPySequence_Base","header",fragment="<stddef.h>")
+%fragment("SwigPySequence_Base","header",fragment="StdTraits")
 {
 %#include <functional>