bpo-40421: Add PyFrame_GetBack() function (GH-19765)

New PyFrame_GetBack() function: get the frame next outer frame.

Replace frame->f_back with PyFrame_GetBack(frame) in most code but
frameobject.c, ceval.c and genobject.c.
diff --git a/Objects/frameobject.c b/Objects/frameobject.c
index 6d288b5..451c895 100644
--- a/Objects/frameobject.c
+++ b/Objects/frameobject.c
@@ -1237,3 +1237,13 @@
     Py_INCREF(code);
     return code;
 }
+
+
+PyFrameObject*
+PyFrame_GetBack(PyFrameObject *frame)
+{
+    assert(frame != NULL);
+    PyFrameObject *back = frame->f_back;
+    Py_XINCREF(back);
+    return back;
+}