Fix types in buffer/memoryview docs (#98118)

The definition of obj in the `Py_buffer` struct is as a PyObject*
ec091bd47e/Include/pybuffer.h (L22)

PyMemoryView_GET_BASE returns `.obj` - thus its return type
should be a PyObject* (or at least a void*). It definitely
doesn't return `Py_buffer`

Doc/c-api/buffer.rst

@@ -99,7 +99,7 @@ a buffer, see :c:func:`PyObject_GetBuffer`.
       For :term:`contiguous` arrays, the value points to the beginning of
       the memory block.
 
-   .. c:member:: void *obj
+   .. c:member:: PyObject *obj
 
       A new reference to the exporting object. The reference is owned by
       the consumer and automatically decremented and set to ``NULL`` by