gh-106023: Remove _PY_FASTCALL_SMALL_STACK constant (#106258)

Remove _PY_FASTCALL_SMALL_STACK constant from the C API: move it to
the internal C API (pycore_call.h).

Author: vstinner

Include/cpython/abstract.h

@@ -2,19 +2,6 @@
 #  error "this header file must not be included directly"
 #endif
 
-/* === Object Protocol ================================================== */
-
-/* Suggested size (number of positional arguments) for arrays of PyObject*
-   allocated on a C stack to avoid allocating memory on the heap memory. Such
-   array is used to pass positional arguments to call functions of the
-   PyObject_Vectorcall() family.
-
-   The size is chosen to not abuse the C stack and so limit the risk of stack
-   overflow. The size is also chosen to allow using the small stack for most
-   function calls of the Python standard library. On 64-bit CPU, it allocates
-   40 bytes on the stack. */
-#define _PY_FASTCALL_SMALL_STACK 5
-
 /* === Vectorcall protocol (PEP 590) ============================= */
 
 // PyVectorcall_NARGS() is exported as a function for the stable ABI.

Include/internal/pycore_call.h

@@ -10,6 +10,18 @@ extern "C" {
 
 #include "pycore_pystate.h"       // _PyThreadState_GET()
 
+/* Suggested size (number of positional arguments) for arrays of PyObject*
+   allocated on a C stack to avoid allocating memory on the heap memory. Such
+   array is used to pass positional arguments to call functions of the
+   PyObject_Vectorcall() family.
+
+   The size is chosen to not abuse the C stack and so limit the risk of stack
+   overflow. The size is also chosen to allow using the small stack for most
+   function calls of the Python standard library. On 64-bit CPU, it allocates
+   40 bytes on the stack. */
+#define _PY_FASTCALL_SMALL_STACK 5
+
+
 // Export for shared stdlib extensions like the math extension,
 // function used via inlined _PyObject_VectorcallTstate() function.
 PyAPI_FUNC(PyObject*) _Py_CheckFunctionResult(