bpo-42519: Replace PyMem_MALLOC() with PyMem_Malloc() (GH-23586)

No longer use deprecated aliases to functions:

* Replace PyMem_MALLOC() with PyMem_Malloc()
* Replace PyMem_REALLOC() with PyMem_Realloc()
* Replace PyMem_FREE() with PyMem_Free()
* Replace PyMem_Del() with PyMem_Free()
* Replace PyMem_DEL() with PyMem_Free()

Modify also the PyMem_DEL() macro to use directly PyMem_Free().
diff --git a/Include/objimpl.h b/Include/objimpl.h
index af53717..464b1bf 100644
--- a/Include/objimpl.h
+++ b/Include/objimpl.h
@@ -102,7 +102,7 @@ PyAPI_FUNC(void *) PyObject_Realloc(void *ptr, size_t new_size);
 PyAPI_FUNC(void) PyObject_Free(void *ptr);
 
 
-/* Macros */
+// Deprecated aliases only kept for backward compatibility.
 #define PyObject_MALLOC         PyObject_Malloc
 #define PyObject_REALLOC        PyObject_Realloc
 #define PyObject_FREE           PyObject_Free
@@ -138,8 +138,8 @@ PyAPI_FUNC(PyVarObject *) _PyObject_NewVar(PyTypeObject *, Py_ssize_t);
 #define PyObject_NewVar(type, typeobj, n) \
                 ( (type *) _PyObject_NewVar((typeobj), (n)) )
 
-// Alias to PyObject_New(). In Python 3.8, PyObject_NEW() called directly
-// PyObject_MALLOC() with _PyObject_VAR_SIZE().
+// Alias to PyObject_NewVar(). In Python 3.8, PyObject_NEW_VAR() called
+// directly PyObject_MALLOC() with _PyObject_VAR_SIZE().
 #define PyObject_NEW_VAR(type, typeobj, n) PyObject_NewVar(type, typeobj, n)