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.