Doc: define PY_SSIZE_T_CLEAN always (GH-12794)

diff --git a/Doc/includes/custom.c b/Doc/includes/custom.c
index fb2c7b2..13d16f5 100644
--- a/Doc/includes/custom.c
+++ b/Doc/includes/custom.c
@@ -1,3 +1,4 @@
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 
 typedef struct {
diff --git a/Doc/includes/custom2.c b/Doc/includes/custom2.c
index 51ab4b8..6477a19 100644
--- a/Doc/includes/custom2.c
+++ b/Doc/includes/custom2.c
@@ -1,3 +1,4 @@
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include "structmember.h"
 
diff --git a/Doc/includes/custom3.c b/Doc/includes/custom3.c
index 09e8735..213d086 100644
--- a/Doc/includes/custom3.c
+++ b/Doc/includes/custom3.c
@@ -1,3 +1,4 @@
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include "structmember.h"
 
diff --git a/Doc/includes/custom4.c b/Doc/includes/custom4.c
index 0994d8f..b0b2906 100644
--- a/Doc/includes/custom4.c
+++ b/Doc/includes/custom4.c
@@ -1,3 +1,4 @@
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include "structmember.h"
 
diff --git a/Doc/includes/run-func.c b/Doc/includes/run-func.c
index 9caf1fd..392f86d 100644
--- a/Doc/includes/run-func.c
+++ b/Doc/includes/run-func.c
@@ -1,3 +1,4 @@
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 
 int
diff --git a/Doc/includes/sublist.c b/Doc/includes/sublist.c
index 376dddf..76ff939 100644
--- a/Doc/includes/sublist.c
+++ b/Doc/includes/sublist.c
@@ -1,3 +1,4 @@
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 
 typedef struct {