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

@@ -1047,6 +1047,7 @@ def test_read_byte_attrs_as_unicode(self):
             actual = open_dataset(tmp_file)
             expected = Dataset(attrs={'foo': 'bar'})
             self.assertDatasetIdentical(expected, actual)
+            actual.close()
 
     def test_encoding_unlimited_dims(self):
         ds = Dataset({'x': ('y', np.arange(10.0))})