Restore _write and _flush in WriteLogger.__setstate__

After unpickling a WriteLogger, calling any log method (msg, info,
debug, etc.) raises AttributeError because __setstate__ only restores
_file and _lock but not _write and _flush.

BytesLogger.__setstate__ already handles this correctly, and so does
WriteLogger.__deepcopy__ — this was just an oversight in __setstate__.

Author: bysiber

src/structlog/_output.py

@@ -186,6 +186,8 @@ def __setstate__(self, state: Any) -> None:
         else:
             self._file = stderr
 
+        self._write = self._file.write
+        self._flush = self._file.flush
         self._lock = _get_lock_for_file(self._file)
 
     def __deepcopy__(self, memodict: dict[str, object]) -> WriteLogger: