Remove pytest.deprecated_call() in TestAssoc::test_unknown

[dlax]

Summary

assoc() no longer raises a deprecation warning since commit 22ae847 but the with pytest.deprecated_call(): in that test was not removed then (in contrast with other test cases).

Maybe this got unnoticed due to a pytest bug?
In any case, using pytest 8+ (and keeping deprecated_call()) shows that no warning is raised and the test fails.

Removing the upper bound on pytest in dev dependencies as tests now pass with pytest 8.0.

Fix #1233.

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[hynek]

This is great – thank you!