Slim down AttrSeries, SparseDataArray

[khaeru]

Since genno was created, some of the upstream packages have seen enhancements that may obviate some of the compatibility code in AttrSeries and SparseDataArray.

To do: investigate each of the following and, where possible, adjust to rely on the upstream functionality / slim down genno itself.

• pandas.Series.attrs exists since possibly as far back as pandas 1.0 but as an "experimental feature that may change without warning".
• pint-pandas, which is still in a beta version (0.6).
• sparse occasionally gets support for additional ufuncs.