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.

Python/Python-ast.c

@@ -4,7 +4,7 @@
 
 #include "Python.h"
 #include "Python-ast.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 
 typedef struct {
     int initialized;

Python/ceval.c

@@ -29,7 +29,6 @@
 #include "opcode.h"
 #include "pydtrace.h"
 #include "setobject.h"
-#include "structmember.h"
 
 #include <ctype.h>
 

Python/context.c

@@ -6,7 +6,7 @@
 #include "pycore_object.h"
 #include "pycore_pyerrors.h"
 #include "pycore_pystate.h"       // _PyThreadState_GET()
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 
 
 #define CONTEXT_FREELIST_MAXLEN 255

Python/hamt.c

@@ -1,8 +1,8 @@
 #include "Python.h"
 
 #include "pycore_hamt.h"
-#include "pycore_object.h"   // _PyObject_GC_TRACK()
-#include "structmember.h"
+#include "pycore_object.h"        // _PyObject_GC_TRACK()
+#include <stddef.h>               // offsetof()
 
 /*
 This file provides an implementation of an immutable mapping using the

Python/structmember.c

@@ -2,8 +2,7 @@
 /* Map C struct members to Python object attributes */
 
 #include "Python.h"
-
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 
 PyObject *
 PyMember_GetOne(const char *addr, PyMemberDef *l)

Python/symtable.c

@@ -2,7 +2,7 @@
 #include "pycore_pystate.h"   // _PyThreadState_GET()
 #include "symtable.h"
 #undef Yield   /* undefine macro conflicting with <winbase.h> */
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 
 /* error strings used for warnings */
 #define GLOBAL_PARAM \

Python/traceback.c

@@ -5,7 +5,7 @@
 
 #include "code.h"
 #include "frameobject.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 #include "osdefs.h"
 #ifdef HAVE_FCNTL_H
 #include <fcntl.h>