bpo-40268: Remove unused structmember.h includes (GH-19530)

If only offsetof() is needed: include stddef.h instead.

When structmember.h is used, add a comment explaining that
PyMemberDef is used.
diff --git a/Objects/rangeobject.c b/Objects/rangeobject.c
index 4bea8d7..751dbb9 100644
--- a/Objects/rangeobject.c
+++ b/Objects/rangeobject.c
@@ -3,7 +3,7 @@
 #include "Python.h"
 #include "pycore_abstract.h"   // _PyIndex_Check()
 #include "pycore_tupleobject.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 
 /* Support objects whose length is > PY_SSIZE_T_MAX.