Try fix: Closes open file (for Windows error)

```
WindowsError: [Error 32] The process cannot access the file because it
is being used by another process
```

xarray/tests/test_backends.py

@@ -1044,10 +1044,9 @@ def test_read_byte_attrs_as_unicode(self):
         with create_tmp_file() as tmp_file:
             with nc4.Dataset(tmp_file, 'w') as nc:
                 nc.foo = b'bar'
-            actual = open_dataset(tmp_file)
-            expected = Dataset(attrs={'foo': 'bar'})
-            self.assertDatasetIdentical(expected, actual)
-            actual.close()
+            with open_dataset(tmp_file) as actual:
+                expected = Dataset(attrs={'foo': 'bar'})
+                self.assertDatasetIdentical(expected, actual)
 
     def test_encoding_unlimited_dims(self):
         ds = Dataset({'x': ('y', np.arange(10.0))})