[TST] Fix test failures for pivot_longer

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• Fix test failures for pivot_longer and test__select_columns

This PR resolves #1154 .

PR Checklist

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Automatic checks

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Relevant Reviewers

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• @ericmjl

[ericmjl]

🚀 Deployed on https://deploy-preview-1155--pyjanitor.netlify.app

[codecov]

Codecov Report

Merging #1155 (968c85d) into dev (a027753) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1155   +/-   ##
=======================================
  Coverage   97.34%   97.34%           
=======================================
  Files          77       77           
  Lines        3240     3240           
=======================================
  Hits         3154     3154           
  Misses         86       86           

[Zeroto521]

these new changes to test_pivot_longer.py passed in my local env (pandas 1.4.1)

[ericmjl]

Merged! 🎉 Thank you, @samukweku!