chore: update to stable snakemake (#250)

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## Summary by CodeRabbit

- **Chores**
- Upgraded a core development dependency to its latest major version,
paving the way for improved performance, enhanced stability, and future
feature enhancements.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: cmeesters

pyproject.toml

@@ -25,7 +25,7 @@ black = "^23.7.0"
 flake8 = "^6.1.0"
 coverage = "^7.3.1"
 pytest = "^8.3.5"
-snakemake = "^8.20.0"
+snakemake = "^9.1.1"
 
 [tool.coverage.run]
 omit = [".*", "*/site-packages/*", "Snakefile"]