Restore _write and _flush in WriteLogger.__setstate__

[bysiber]

WriteLogger.__setstate__ restores _file and _lock but not _write and _flush. After unpickling, any log method raises AttributeError:

import pickle
from structlog import WriteLogger

logger = pickle.loads(pickle.dumps(WriteLogger()))
logger.info("hello")
# AttributeError: 'WriteLogger' object has no attribute '_write'

BytesLogger.__setstate__ already restores all four attributes correctly, and WriteLogger.__deepcopy__ does too — this was just missed in __setstate__.

The fix adds the two missing assignments after restoring _file.

[RonnyPfannschmidt]

@hynek heads up , this one has been running a AI DOS on pytest and pluggy reviewer capacity