bpo-39487: Merge duplicated _Py_IDENTIFIER identifiers in C code (GH-18254)

Moving repetitive `_Py_IDENTIFIER` instances to a global location helps identify them more easily in regards to sub-interpreter support.
diff --git a/Objects/rangeobject.c b/Objects/rangeobject.c
index 9311f8b..e716820 100644
--- a/Objects/rangeobject.c
+++ b/Objects/rangeobject.c
@@ -18,6 +18,8 @@
     PyObject *length;
 } rangeobject;
 
+_Py_IDENTIFIER(iter);
+
 /* Helper function for validating step.  Always returns a new reference or
    NULL on error.
 */
@@ -757,7 +759,6 @@
 static PyObject *
 rangeiter_reduce(rangeiterobject *r, PyObject *Py_UNUSED(ignored))
 {
-    _Py_IDENTIFIER(iter);
     PyObject *start=NULL, *stop=NULL, *step=NULL;
     PyObject *range;
 
@@ -915,7 +916,6 @@
 static PyObject *
 longrangeiter_reduce(longrangeiterobject *r, PyObject *Py_UNUSED(ignored))
 {
-    _Py_IDENTIFIER(iter);
     PyObject *product, *stop=NULL;
     PyObject *range;