bpo-42260: Add _PyInterpreterState_SetConfig() (GH-23158)

* Inline _PyInterpreterState_SetConfig(): replace it with
  _PyConfig_Copy().
* Add _PyErr_SetFromPyStatus()
* Add _PyInterpreterState_GetConfigCopy()
* Add a new _PyInterpreterState_SetConfig() function.
* Add an unit which gets, modifies, and sets the config.

Include/cpython/pystate.h

@@ -193,6 +193,36 @@ PyAPI_FUNC(void) _PyInterpreterState_SetEvalFrameFunc(
 
 PyAPI_FUNC(const PyConfig*) _PyInterpreterState_GetConfig(PyInterpreterState *interp);
 
+/* Get a copy of the current interpreter configuration.
+
+   Return 0 on success. Raise an exception and return -1 on error.
+
+   The caller must initialize 'config', using PyConfig_InitPythonConfig()
+   for example.
+
+   Python must be preinitialized to call this method.
+   The caller must hold the GIL. */
+PyAPI_FUNC(int) _PyInterpreterState_GetConfigCopy(
+    struct PyConfig *config);
+
+/* Set the configuration of the current interpreter.
+
+   This function should be called during or just after the Python
+   initialization.
+
+   Update the sys module with the new configuration. If the sys module was
+   modified directly after the Python initialization, these changes are lost.
+
+   Some configuration like faulthandler or warnoptions can be updated in the
+   configuration, but don't reconfigure Python (don't enable/disable
+   faulthandler and don't reconfigure warnings filters).
+
+   Return 0 on success. Raise an exception and return -1 on error.
+
+   The configuration should come from _PyInterpreterState_GetConfigCopy(). */
+PyAPI_FUNC(int) _PyInterpreterState_SetConfig(
+    const struct PyConfig *config);
+
 // Get the configuration of the currrent interpreter.
 // The caller must hold the GIL.
 PyAPI_FUNC(const PyConfig*) _Py_GetConfig(void);

Include/internal/pycore_initconfig.h

@@ -44,6 +44,8 @@ struct pyruntimestate;
 #define _PyStatus_UPDATE_FUNC(err) \
     do { err.func = _PyStatus_GET_FUNC(); } while (0)
 
+PyObject* _PyErr_SetFromPyStatus(PyStatus status);
+
 /* --- PyWideStringList ------------------------------------------------ */
 
 #define _PyWideStringList_INIT (PyWideStringList){.length = 0, .items = NULL}

Include/internal/pycore_interp.h

@@ -263,13 +263,7 @@ struct _is {
     struct ast_state ast;
 };
 
-/* Used by _PyImport_Cleanup() */
 extern void _PyInterpreterState_ClearModules(PyInterpreterState *interp);
-
-extern PyStatus _PyInterpreterState_SetConfig(
-    PyInterpreterState *interp,
-    const PyConfig *config);
-
 extern void _PyInterpreterState_Clear(PyThreadState *tstate);