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ARROW-16174: [Python] Fix FixedSizeListArray.flatten() on sliced input (#14000)

[ARROW-16174](https://issues.apache.org/jira/browse/ARROW-16174)

Current behavior
```python
import pyarrow as pa

array = pa.array([[1], [2], [3]], type=pa.list_(pa.int64(), list_size=1))
array[2:].flatten().to_pylist()
[1, 2, 3]
```

After this patch
```python
import pyarrow as pa

array = pa.array([[1], [2], [3]], type=pa.list_(pa.int64(), list_size=1))
array[2:].flatten().to_pylist()
[3]
```

Authored-by: Miles Granger <miles59923@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>