ci: python version (#20)

- Relax python version constraints to fix `pelican` and `flowo` plugins
- Downgrade `sphinx` since `v8.3.0` was deemed a [bad
release](https://pypi.org/project/Sphinx/8.3.0/)

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

* **Chores**
* Updated Python version dependency specification to support a wider
range of compatible versions.

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

Author: fgvieira