WIP: Modernizing setup.

Summary

Replacing long deprecated tests_requires by a test extra.

Focus

Does the pipeline pass?

Test Environment

Pipeline.

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• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
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• I understand the intent of this MR
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• Appropriate user and developer documentation (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
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CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added ###
 
+* New setup extra `test` which installs the dependencies for testing.
+
 ### Changed ###
 
 ### Deprecated ###