What's new in 2.1.1 (September XX, 2023)

These are the changes in pandas 2.1.1. See :ref:`release` for a full changelog including other versions of pandas.

Fixed regressions

Fixed regression in :func:`concat` when :class:`DataFrame` 's have two different extension dtypes (:issue:`54848`)
Fixed regression in :func:`merge` when merging over a PyArrow string index (:issue:`54894`)
Fixed regression in :func:`read_csv` when usecols is given and dtypes is a dict for engine="python" (:issue:`54868`)
Fixed regression in :func:`read_csv` when delim_whitespace is True (:issue:`54918`, :issue:`54931`)
Fixed regression in :meth:`.GroupBy.get_group` raising for axis=1 (:issue:`54858`)
Fixed regression in :meth:`DataFrame.__setitem__` raising AssertionError when setting a :class:`Series` with a partial :class:`MultiIndex` (:issue:`54875`)
Fixed regression in :meth:`DataFrame.filter` not respecting the order of elements for filter (:issue:`54980`)
Fixed regression in :meth:`DataFrame.to_sql` not roundtripping datetime columns correctly for sqlite (:issue:`54877`)
Fixed regression in :meth:`MultiIndex.append` raising when appending overlapping :class:`IntervalIndex` levels (:issue:`54934`)
Fixed regression in :meth:`Series.drop_duplicates` for PyArrow strings (:issue:`54904`)
Fixed regression in :meth:`Series.interpolate` raising when fill_value was given (:issue:`54920`)
Fixed regression in :meth:`Series.value_counts` raising for numeric data if bins was specified (:issue:`54857`)
Fixed regression when comparing a :class:`Series` with datetime64 dtype with None (:issue:`54870`)

Fixed bug for :class:`ArrowDtype` raising NotImplementedError for fixed-size list (:issue:`55000`)
Fixed bug in :meth:`DataFrame.stack` with future_stack=True and columns a non-:class:`MultiIndex` consisting of tuples (:issue:`54948`)