Add PyErr_WarnEx() so C code can pass the stacklevel to warnings.warn(). This provides the proper warning for struct.pack(). PyErr_Warn() is now deprecated in favor of PyErr_WarnEx(). As mentioned by Tim Peters on python-dev.

#include "Python.h"

#ifndef PLATFORM
#define PLATFORM "unknown"
#endif

const char *
Py_GetPlatform(void)
{
	return PLATFORM;
}