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/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 7f39022..938df24 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -439,7 +439,7 @@
     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
@@ -3632,7 +3632,8 @@
         /* Before _PyUnicode_InitEncodings() is called, the Python codec
            machinery is not ready and so cannot be used:
            use wcstombs() in this case. */
-        const wchar_t *filesystem_errors = interp->config.filesystem_errors;
+        const PyConfig *config = _PyInterpreterState_GetConfig(interp);
+        const wchar_t *filesystem_errors = config->filesystem_errors;
         assert(filesystem_errors != NULL);
         _Py_error_handler errors = get_error_handler_wide(filesystem_errors);
         assert(errors != _Py_ERROR_UNKNOWN);
@@ -3868,7 +3869,8 @@
         /* Before _PyUnicode_InitEncodings() is called, the Python codec
            machinery is not ready and so cannot be used:
            use mbstowcs() in this case. */
-        const wchar_t *filesystem_errors = interp->config.filesystem_errors;
+        const PyConfig *config = _PyInterpreterState_GetConfig(interp);
+        const wchar_t *filesystem_errors = config->filesystem_errors;
         assert(filesystem_errors != NULL);
         _Py_error_handler errors = get_error_handler_wide(filesystem_errors);
         assert(errors != _Py_ERROR_UNKNOWN);
@@ -15894,7 +15896,7 @@
 init_stdio_encoding(PyThreadState *tstate)
 {
     /* Update the stdio encoding to the normalized Python codec name. */
-    PyConfig *config = &tstate->interp->config;
+    PyConfig *config = (PyConfig*)_PyInterpreterState_GetConfig(tstate->interp);
     if (config_get_codec_name(&config->stdio_encoding) < 0) {
         return _PyStatus_ERR("failed to get the Python codec name "
                              "of the stdio encoding");
@@ -15906,7 +15908,7 @@
 static int
 init_fs_codec(PyInterpreterState *interp)
 {
-    PyConfig *config = &interp->config;
+    const PyConfig *config = _PyInterpreterState_GetConfig(interp);
 
     _Py_error_handler error_handler;
     error_handler = get_error_handler_wide(config->filesystem_errors);
@@ -15964,7 +15966,7 @@
     /* Update the filesystem encoding to the normalized Python codec name.
        For example, replace "ANSI_X3.4-1968" (locale encoding) with "ascii"
        (Python codec name). */
-    PyConfig *config = &interp->config;
+    PyConfig *config = (PyConfig*)_PyInterpreterState_GetConfig(interp);
     if (config_get_codec_name(&config->filesystem_encoding) < 0) {
         _Py_DumpPathConfig(tstate);
         return _PyStatus_ERR("failed to get the Python codec "
@@ -16008,7 +16010,7 @@
 _PyUnicode_EnableLegacyWindowsFSEncoding(void)
 {
     PyInterpreterState *interp = _PyInterpreterState_GET_UNSAFE();
-    PyConfig *config = &interp->config;
+    PyConfig *config = (PyConfig *)_PyInterpreterState_GetConfig(interp);
 
     /* Set the filesystem encoding to mbcs/replace (PEP 529) */
     wchar_t *encoding = _PyMem_RawWcsdup(L"mbcs");