REF: Groupby._get_cythonized_result operate blockwise in axis==1 case (#43435)

Author: jbrockmendel

doc/source/whatsnew/v1.3.4.rst

@@ -23,8 +23,7 @@ Fixed regressions
 
 Bug fixes
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+- Fixed bug in :meth:`.GroupBy.mean` with datetimelike values including ``NaT`` values returning incorrect results (:issue`:43132`)
 
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