chore: migrate to pytest-doctestplus

[Saransh-cpp]

Description

See Erotemic/xdoctest#151

Checklist

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't any other open Pull Requests for the required change?
• Does your submission pass pre-commit? ($ pre-commit run --all-files or $ nox -s lint)
• Does your submission pass tests? ($ pytest or $ nox -s tests)
• Does the documentation build with your changes? ($ cd docs; make clean; make html or $ nox -s docs)
• Does your submission pass the doctests? ($ xdoctest ./src/vector or $ nox -s doctests)

Before Merging

• Summarize the commit messages into a brief review of the Pull request.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ab111e0) 83.21% compared to head (8b9149a) 82.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #416      +/-   ##
==========================================
- Coverage   83.21%   82.26%   -0.96%     
==========================================
  Files          96       96              
  Lines       11429    11429              
==========================================
- Hits         9511     9402     -109     
- Misses       1918     2027     +109     

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