[BUG] Pin pyyaml dependency due to cython>=3 incompatibility

[peytondmurray]

Fixes #915.

Description

This pull request pins pyyaml==5.3.1 due to an incompatibility with cython>=3, which is a transitive dependency of this project.

Pull request checklist

• [ ] Did you test this change locally?
• [x] Did you update the documentation (if required)?
• [ ] Did you add/update relevant tests for this change (if required)?

@soapy1 Would you be willing to check that this works for you? It's enough to run tests and then hatch shell dev inside conda-store-server/.

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