GH-131238: More refactoring of core header files (GH-131351)

Adds new pycore_stats.h header file to help break dependencies involving the pycore_code.h header.

Objects/genobject.c

@@ -17,8 +17,6 @@
 #include "pycore_pyerrors.h"      // _PyErr_ClearExcState()
 #include "pycore_pystate.h"       // _PyThreadState_GET()
 
-#include "pystats.h"
-
 // Forward declarations
 static PyObject* gen_close(PyObject *, PyObject *);
 static PyObject* async_gen_asend_new(PyAsyncGenObject *, PyObject *);