bpo-30061: Check if PyObject_Size()/PySequence_Size()/PyMapping_Size() (#1096)

raised an error.

Replace them with using concrete types API that never fails if appropriate.
diff --git a/Modules/_elementtree.c b/Modules/_elementtree.c
index 456c4a2..82df58f 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -1878,7 +1878,7 @@
                 );
             return -1;
         }
-        newlen = PySequence_Size(seq);
+        newlen = PySequence_Fast_GET_SIZE(seq);
 
         if (step !=  1 && newlen != slicelen)
         {
@@ -3660,7 +3660,7 @@
         return NULL;
     }
 
-    for (i = 0; i < PySequence_Size(events_seq); ++i) {
+    for (i = 0; i < PySequence_Fast_GET_SIZE(events_seq); ++i) {
         PyObject *event_name_obj = PySequence_Fast_GET_ITEM(events_seq, i);
         const char *event_name = NULL;
         if (PyUnicode_Check(event_name_obj)) {