Fix version incompatibility with pytest and pytest-doctestplus

[ethanwharris]

What does this PR do?

Pytest == 7.0 with doctest-plus causes erros in Python 3.6.

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #827 (37a1dd7) into master (ac6539b) will decrease coverage by 24%.
The diff coverage is n/a.

❗ Current head 37a1dd7 differs from pull request most recent head 6e885a3. Consider uploading reports for the commit 6e885a3 to get more accurate results

@@           Coverage Diff           @@
##           master   #827     +/-   ##
=======================================
- Coverage      95%    71%    -24%     
=======================================
  Files         166    166             
  Lines        6832   6832             
=======================================
- Hits         6457   4817   -1640     
- Misses        375   2015   +1640