bpo-40268: Add _PyInterpreterState_GetConfig() (GH-19492)

Don't access PyInterpreterState.config member directly anymore, but
use new functions:

* _PyInterpreterState_GetConfig()
* _PyInterpreterState_SetConfig()
* _Py_GetConfig()
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 92d6faa..492988e 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -9,6 +9,7 @@
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
 #include "pycore_object.h"
+#include "pycore_pystate.h"
 #include "structmember.h"
 #include "_iomodule.h"
 
@@ -996,7 +997,7 @@
     PyInterpreterState *interp = _PyInterpreterState_GET_UNSAFE();
 #ifndef Py_DEBUG
     /* In release mode, only check in development mode (-X dev) */
-    if (!interp->config.dev_mode) {
+    if (!_PyInterpreterState_GetConfig(interp)->dev_mode) {
         return 0;
     }
 #else