bpo-34485: Add _Py_ClearStandardStreamEncoding() (GH-8982)

* Move Py_SetStandardStreamEncoding() from pylifecycle.c
  to coreconfig.c
* Add _Py_ClearStandardStreamEncoding() private function.
* pymain_free() now calls _Py_ClearStandardStreamEncoding().
* Add assertions add the end of _PyCoreConfig_Read()
* _PyCoreConfig_Copy(): rename COPY_STR_ATTR() macro
  to COPY_WSTR_ATTR().
* Fix get_stdio_errors() indentation.