bpo-33738: Fix macros which contradict PEP 384

[ctismer]

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 sections 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