CLN: simplify _shallow_copy
The class hierarchy of in/around groupby is really tough, this simplifies it a little bit
Is `GroupByMixin` used at all within groupby? Maybe worth moving the entire class huh, looks like it isnt.  any strong opinion on where it would be moved? Maybe merge into WindowMixin. If it’s not actually used for GroupBy I don’t even think we need to keep the GroupByMixin name as that would be rather misleading 

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> On Nov 7, 2019, at 5:14 PM, jbrockmendel <notifications@github.com> wrote:
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> huh, looks like it isnt. any strong opinion on where it would be moved?
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 let me look 

I made these for consistency between group by and window ops > Maybe merge into WindowMixin. If it’s not actually used for GroupBy I don’t even think we need to keep the GroupByMixin name as that would be rather misleading

For now at least I'm leaning towards keeping it where it is.  it would need to go into some shared-by-resample/window location, which describes a lot of groupby code anyway. thxs