[DEV] Support python3 in merge_livy_pr script

[mgaido91]

What changes were proposed in this pull request?

The merge_livy_prscript supports only python 2, which is nowadays not maintained anymore and not even usable in some environments. The PR updates the script so that it support execution on python3 as well.

How was this patch tested?

Manual execution to merge PR #416.

[codecov-commenter]

Codecov Report

Merging #418 (b467d64) into master (5dccc47) will decrease coverage by 36.82%. Report is 1 commits behind head on master. The diff coverage is n/a.

@@              Coverage Diff              @@
##             master     #418       +/-   ##
=============================================
- Coverage     65.50%   28.68%   -36.82%     
+ Complexity      952      379      -573     
=============================================
  Files           103      103               
  Lines          6062     6062               
  Branches        916      916               
=============================================
- Hits           3971     1739     -2232     
- Misses         1542     3971     +2429     
+ Partials        549      352      -197     

see 87 files with indirect coverage changes

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