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/textio.c b/Modules/_io/textio.c
index 9e33c1e..1abc9ca 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -8,10 +8,10 @@
 
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
-#include "pycore_interp.h"      // PyInterpreterState.fs_codec
+#include "pycore_interp.h"        // PyInterpreterState.fs_codec
 #include "pycore_object.h"
-#include "pycore_pystate.h"     // _PyInterpreterState_GET()
-#include "structmember.h"
+#include "pycore_pystate.h"       // _PyInterpreterState_GET()
+#include "structmember.h"         // PyMemberDef
 #include "_iomodule.h"
 
 /*[clinic input]