Attempt to fix indexing for Dask

This is a naive attempt to make `isel` work with Dask

Known limitation: it triggers the computation.

xarray/core/indexing.py

@@ -5,7 +5,7 @@
 from contextlib import suppress
 from datetime import timedelta
 from typing import Any, Callable, Iterable, List, Optional, Tuple, Union
-import  dask.array as da
+import dask.array as da
 
 import numpy as np
 import pandas as pd