DOC: Fixed flake8 issues in whatsnew v0.13.* pandas-dev#24256

Added setup.cfg

Author: Seuss27

setup.cfg

@@ -117,7 +117,7 @@ known_post_core=pandas.tseries,pandas.io,pandas.plotting
 sections=FUTURE,STDLIB,THIRDPARTY,PRE_CORE,DTYPES,FIRSTPARTY,POST_CORE,LOCALFOLDER
 
 known_first_party=pandas
-known_third_party=Cython,numpy,python-dateutil,pytz,pyarrow,pytest
+known_third_party=Cython,numpy,dateutil,python-dateutil,pytz,pyarrow,pytest
 multi_line_output=4
 force_grid_wrap=0
 combine_as_imports=True