ENH: Deprecate non-keyword arguments for drop_duplicates.

Author: jmholzer

doc/source/whatsnew/v1.3.0.rst

@@ -647,7 +647,7 @@ Deprecations
 - Deprecated setting :attr:`Categorical._codes`, create a new :class:`Categorical` with the desired codes instead (:issue:`40606`)
 - Deprecated behavior of :meth:`DatetimeIndex.union` with mixed timezones; in a future version both will be cast to UTC instead of object dtype (:issue:`39328`)
 - Deprecated using ``usecols`` with out of bounds indices for ``read_csv`` with ``engine="c"`` (:issue:`25623`)
-- Deprecated passing arguments as positional in :meth:`DataFrame.drop_duplicates` and :meth:`Series.drop_duplicates` (:issue:`41485`)
+- Deprecated passing arguments as positional in :meth:`DataFrame.drop_duplicates` (except for ``subset``) and :meth:`Series.drop_duplicates` (:issue:`41485`)
 
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