fixed outdated pytest tardis command in documentation

[afloers]

The documentation currently contains the old python setup.py test command.
This PR replaces it with the new pytest tardis command.

How has this been tested?

• Testing pipeline.
• Other.

Examples

Type of change

• Bug fix.
• New feature.
• Breaking change.
• None of the above.

Checklist

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
  • (optional) I have built the documentation on my fork following the instructions.
• I have assigned and requested two reviewers for this pull request.

[codecov]

Codecov Report

Merging #1876 (4dc3fb2) into master (c44183d) will not change coverage.
The diff coverage is n/a.

❗ Current head 4dc3fb2 differs from pull request most recent head 7e2ca39. Consider uploading reports for the commit 7e2ca39 to get more accurate results

@@           Coverage Diff           @@
##           master    #1876   +/-   ##
=======================================
  Coverage   62.24%   62.24%           
=======================================
  Files          67       67           
  Lines        6830     6830           
=======================================
  Hits         4251     4251           
  Misses       2579     2579           

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