_get_attributes_dict formalization

[jbrockmendel]

Index subclasses have a _get_attributes_dict method that would be useful to employ elsewhere (here I'm thinking of DateOffset subclasses and discussion in #19174). Could/should that be defined in a Mixin class (say in core.base) so that pattern could be documented and used elsewhere?

[jreback]

we do things this also for groupby/rolling as well. I suppose it could be moved to the pandas/core/base.py/PandasObject which already mixes everything (well except for offsets)

[jbrockmendel]

Because offsets are perf-sensitive it might need to be implemented in cython. That aside, rolling it into PandasObject makes a lot of sense.

Block.make_block looks a lot like Index._shallow_copy. A bunch of the DatetimeIndexOpsMixin arithmetic/comparison (ones that I think @TomAugspurger agrees would go nicely in an Array-level class) use _shallow_copy. Making the Block classes conform to the IndexOps name conventions would make that refactor a lot easier.

[jreback]

not sure we need an issue for this. if you want to do it great. Not sure make_block and _shallow_copy make sense to be combined, they are different animals.