Issue #13959: Keep imp.get_magic() in C code, but cache in importlib for performance. While get_magic() could move to Lib/imp.py, having to support PyImport_GetMagicNumber() would lead to equal, if not more, C code than sticking with the status quo.

#include "Python.h"

#ifndef PLATFORM
#define PLATFORM "unknown"
#endif

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