bpo-27867: Expand the PySlice_GetIndicesEx macro. (#1023) (#1046)

(cherry picked from commit b879fe8)
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 0dc4805..4f4c959 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -784,10 +784,10 @@
     else if (PySlice_Check(item)) {
         Py_ssize_t start, stop, step, slicelen;
 
-        if (PySlice_GetIndicesEx((PySliceObject *)item, self->size,
-                         &start, &stop, &step, &slicelen) < 0) {
+        if (_PySlice_Unpack((PySliceObject *)item, &start, &stop, &step) < 0) {
             return NULL;
         }
+        slicelen = _PySlice_AdjustIndices(self->size, &start, &stop, step);
 
         if (slicelen <= 0)
             return PyString_FromStringAndSize("", 0);
@@ -939,11 +939,10 @@
     else if (PySlice_Check(item)) {
         Py_ssize_t start, stop, step, slicelen;
 
-        if (PySlice_GetIndicesEx((PySliceObject *)item,
-                                 self->size, &start, &stop,
-                                 &step, &slicelen) < 0) {
+        if (_PySlice_Unpack((PySliceObject *)item, &start, &stop, &step) < 0) {
             return -1;
         }
+        slicelen = _PySlice_AdjustIndices(self->size, &start, &stop, step);
         if (value == NULL) {
             PyErr_SetString(PyExc_TypeError,
                 "mmap object doesn't support slice deletion");