Temporarily add back Index._get_attributes_dict for dask compat
See https://github.com/pandas-dev/pandas/pull/43895#issuecomment-938472299. There was of course no need to revert the full PR, just to add this method again. 
(failure is an unrelated "unexpected ResourceWarning") IIUC the motivation is for dask.  Are dask users affected or just their CI?  If the latter, seems easier for them to fix than for revert-fix-unrevert?

If the former, can you add a FutureWarning on _get_attributes_dict It's also dask users/developers, _if_ they are using a development version of pandas. The current situation basically makes you can't test/develop in a dask context with pandas master at the moment. Adding this back temporarily is an easy way to make this process a bit more smooth.

> If the former, can you add a FutureWarning on _get_attributes_dict

I don't think that's needed. Users are not directly use this, but through dask. 
(but of course easy to add) > (but of course easy to add)

yah this is the easiest way to get me to "OK sure" Added a warning.