Fix output tests for core and pin tabulate version

[troydai]

Brief:

1. Fix tabulate to 0.7.7 and remove it from requirements.txt.
2. Fix the output test in Core.

[codecov-io]

Codecov Report

Merging #3313 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3313   +/-   ##
=======================================
  Coverage   70.32%   70.32%           
=======================================
  Files         391      391           
  Lines       25110    25110           
  Branches     3815     3815           
=======================================
  Hits        17658    17658           
  Misses       6336     6336           
  Partials     1116     1116

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