TST: assert message shows unnecessary diff

[sinhrks]

• passes git diff upstream/master | flake8 --diff

removed unnecessary trailing None.

pd.util.testing.assert_index_equal(pd.Index([1, 2, 3]), pd.Index([1 ,2, 4]))
# AssertionError: Index are different
# 
# Index values are different (33.33333 %)
# [left]:  Int64Index([1, 2, 3], dtype='int64')
# [right]: Int64Index([1, 2, 4], dtype='int64')None

[codecov-io]

Current coverage is 84.38%

Merging #13676 into master will increase coverage by <.01%

@@             master     #13676   diff @@
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  Files           142        142          
  Lines         51223      51223          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43223      43224     +1   
+ Misses         8000       7999     -1   
  Partials          0          0          

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[jreback]

yes this was annoying!