bpo-31890: define METH_STACKLESS (#4159)

Add METH_STACKLESS to prevent future collisions.

Include/methodobject.h

@@ -89,7 +89,16 @@ PyAPI_FUNC(PyObject *) PyCFunction_NewEx(PyMethodDef *, PyObject *,
 
 #ifndef Py_LIMITED_API
 #define METH_FASTCALL  0x0080
+#endif
 
+/* This bit is preserved for Stackless Python */
+#ifdef STACKLESS
+#define METH_STACKLESS 0x0100
+#else
+#define METH_STACKLESS 0x0000
+#endif
+
+#ifndef Py_LIMITED_API
 typedef struct {
     PyObject_HEAD
     PyMethodDef *m_ml; /* Description of the C function to call */