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

Updated setup.cfg to the most recent version before PR

Author: Seuss27

setup.cfg

@@ -45,7 +45,6 @@ ignore = E402,  # module level import not at top of file
          E703,  # statement ends with a semicolon
 
 exclude =
-    doc/source/whatsnew/v0.12.0.rst
     doc/source/whatsnew/v0.15.0.rst
     doc/source/whatsnew/v0.15.1.rst
     doc/source/whatsnew/v0.15.2.rst
@@ -117,7 +116,7 @@ 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,dateutil,python-dateutil,pytz,pyarrow,pytest
+known_third_party=Cython,numpy,dateutil,matplotlib,python-dateutil,pytz,pyarrow,pytest
 multi_line_output=4
 force_grid_wrap=0
 combine_as_imports=True