update tabulate version to 0.8.6

requirements.txt

@@ -2,5 +2,5 @@ numpy >= 1.17.0
 scipy >= 1.3.3
 decorator >= 4.4.0
 pytest >= 5.4.3
-tabulate >= 0.8.5  # for auto-tuning
+tabulate >= 0.8.6  # for auto-tuning
 xgboost >= 0.9     # for auto-tuning