DEPR: deprecate cols / change to subset in duplicated/drop_duplicates

[jreback]

for consistency with dropna (and also to not use cols which is being deprecated)

related #6645

[jsexauer]

In working on this, the only unit tests I could find for duplicated where in pandas/tseries/tests/test_timeseries.py. Shouldn't there be some tests in pandas/tests/test_frame.py? Regardless, where would you like to see the unit test the verifies the warnings are errors are raised as expected?

[jsexauer]

Also, can I please assigned to this so I can convert this issue into a pr? Thanks.

[jreback]

go ahead

hmm should be some tests for duplicated/drop_duplicates in test_frame

if not prob just the underlying routines see tested

pls add a suite of tests (including the depr warning tests)

thanks

[jreback]

closed by #6693