We can't use expanding-code-block in docstrings (#909)

It introduces a downstream dependency on our extension

Author: Cadair

ndcube/ndcube.py

@@ -1550,8 +1550,7 @@ def to_nddata(self,
 
         Examples
         --------
-        .. expanding-code-block:: python
-          :summary: Expand to see cube instantiated.
+        .. testsetup::
 
           >>> import astropy.units as u
           >>> import astropy.wcs