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/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index f4261b3..2468f45 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -1,6 +1,6 @@
#include "Python.h"
#include "pycore_object.h"
-#include "structmember.h" /* for offsetof() */
+#include <stddef.h> // offsetof()
#include "_iomodule.h"
/*[clinic input]