DEPR: deprecate obj argument in GroupBy.get_group

[natmokval]

• closes DEPR: obj argument in GroupBy.get_group #53545
• All code checks passed.

obj argument was deprecated in GroupBy.get_group and removed from the parameters in GroupBy.get_group docs.

[natmokval]

@rhshadrach, could you please take a look at this pr?

[DeaMariaLeon]

Small issue: I think that it returns Series or DataFrame

[natmokval]

thanks, I rolled back my changes there completely.

[rhshadrach]

Thanks for the PR! This is indeed how the code should look after the deprecation is enforced. However we need to first introduce the deprecation so as to not break any user code but rather warn them about the change. If you're unfamiliar with this, I would suggest looking at merged PRs with the "Deprecate" label to get a better sense as to what this entails.

[natmokval]

Thank you, @rhshadrach for the comment. Sorry, my fault, I corrected my pr. Could you please take a look at update.

[rhshadrach]

Looks good; could you add a test for the warning? I'd recommend placing it in tests/groupby/test_groupby.py.

[rhshadrach]

lgtm

[natmokval]

Thank you @rhshadrach for reviewing this. Sorry for the delay with adding the test.