Parallelize testing to speed up CI

[matthewfeickert]

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Summary

The CI is a bit slow at the moment (most runs take over 18 minutes to complete). Parallelizing the testing with pytest-split would help and could potentially bring things down below 10 minutes.

Note that pytest-split is an individual's project, and using the pytest-dev supported pytest-xdist is maybe a better long term plan.

Additional Information

The idea of using pytest-split comes from searching and finding the 2020 blog Blazing fast CI with pytest-split and GitHub Actions which demos the usage in the example project https://github.com/jerry-git/pytest-split-gh-actions-demo.

Code of Conduct

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[matthewfeickert]

Hynek is trying to get this working for attrs (https://github.com/python-attrs/attrs/pull/1011) so hopefully he has some luck there that we can copy in the future.