TST: register slow marker (#16797)

* TST: register slow marker

* Update setup.cfg

Author: ri938

doc/source/whatsnew/v0.20.3.txt

@@ -43,6 +43,7 @@ Bug Fixes
 - Fixed a bug in failing to compute rolling computations of a column-MultiIndexed ``DataFrame`` (:issue:`16789`, :issue:`16825`)
 - Bug in a DataFrame/Series with a ``TimedeltaIndex`` when slice indexing (:issue:`16637`)
 - Bug in reindexing on an empty ``CategoricalIndex`` (:issue:`16770`)
+- Fixed a pytest marker failing downstream packages' tests suites (:issue:`16680`)
 
 Conversion
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