bpo-33738: Fix macros which contradict PEP 384 (GH-7477)

During development of the limited API support for PySide,
we saw an error in a macro that accessed a type field.

This patch fixes the 7 errors in the Python headers.
Macros which were not written as capitals were implemented
as function.

To do the necessary analysis again, a script was included that
parses all headers and looks for "->tp_" in serctions which can
be reached with active limited API.

It is easily possible to call this script as a test.

Error listing:

../../Include/objimpl.h:243
#define PyObject_IS_GC(o) (PyType_IS_GC(Py_TYPE(o)) && \
    (Py_TYPE(o)->tp_is_gc == NULL || Py_TYPE(o)->tp_is_gc(o)))
Action: commented only

../../Include/objimpl.h:362
#define PyType_SUPPORTS_WEAKREFS(t) ((t)->tp_weaklistoffset > 0)
Action: commented only

../../Include/objimpl.h:364
#define PyObject_GET_WEAKREFS_LISTPTR(o) \
    ((PyObject **) (((char *) (o)) + Py_TYPE(o)->tp_weaklistoffset))
Action: commented only

../../Include/pyerrors.h:143
#define PyExceptionClass_Name(x) \
     ((char *)(((PyTypeObject*)(x))->tp_name))
Action: implemented function

../../Include/abstract.h:593
#define PyIter_Check(obj) \
    ((obj)->ob_type->tp_iternext != NULL && \
     (obj)->ob_type->tp_iternext != &_PyObject_NextNotImplemented)
Action: implemented function

../../Include/abstract.h:713
#define PyIndex_Check(obj)                              \
    ((obj)->ob_type->tp_as_number != NULL &&            \
     (obj)->ob_type->tp_as_number->nb_index != NULL)
Action: implemented function

../../Include/abstract.h:924
#define PySequence_ITEM(o, i)\
    ( Py_TYPE(o)->tp_as_sequence->sq_item(o, i) )
Action: commented only

Objects/abstract.c

@@ -1244,6 +1244,14 @@ PyNumber_Absolute(PyObject *o)
     return type_error("bad operand type for abs(): '%.200s'", o);
 }
 
+#undef PyIndex_Check
+int
+PyIndex_Check(PyObject *obj)
+{
+    return obj->ob_type->tp_as_number != NULL &&
+           obj->ob_type->tp_as_number->nb_index != NULL;
+}
+
 /* Return a Python int from the object item.
    Raise TypeError if the result is not an int
    or if the object cannot be interpreted as an index.
@@ -2535,6 +2543,13 @@ PyObject_GetIter(PyObject *o)
     }
 }
 
+#undef PyIter_Check
+int PyIter_Check(PyObject *obj)
+{
+    return obj->ob_type->tp_iternext != NULL &&
+           obj->ob_type->tp_iternext != &_PyObject_NextNotImplemented;
+}
+
 /* Return next item.
  * If an error occurs, return NULL.  PyErr_Occurred() will be true.
  * If the iteration terminates normally, return NULL and clear the

Objects/exceptions.c

@@ -342,6 +342,12 @@ PyException_SetContext(PyObject *self, PyObject *context)
     Py_XSETREF(((PyBaseExceptionObject *)self)->context, context);
 }
 
+#undef PyExceptionClass_Name
+char *
+PyExceptionClass_Name(PyObject *ob)
+{
+    return ((PyTypeObject*)ob)->tp_name;
+}
 
 static struct PyMemberDef BaseException_members[] = {
     {"__suppress_context__", T_BOOL,