chore: update to stable snakemake (#142)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Chores**
- Updated the dependency specification for the `snakemake` package to
ensure a stable version (8.20.0) is used, enhancing the reliability of
the environment.

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

pyproject.toml

@@ -25,7 +25,7 @@ black = "^23.7.0"
 flake8 = "^6.1.0"
 coverage = "^7.3.1"
 pytest = "^7.4.2"
-snakemake = {git = "https://github.com/snakemake/snakemake.git"}
+snakemake = "^8.20.0"
 
 [tool.coverage.run]
 omit = [".*", "*/site-packages/*", "Snakefile"]