API: Add Series/Index.unique(dropna=True)

[sinhrks]

Series/Index.nunique has dropna kw to exclude NaN. Add the same kw to .unique also.

As #13979 changes .unique to always return Index, we can replace Index._get_unique_index(dropna) if this option is added.

[jbrockmendel]

Why isn't obj.unique().dropna() not good enough?

[mroeschke]

Yeah agreed calling dropna should be sufficient. Closing