DevOps improvement and standardization

This project focuses on improving the development and testing infrastructure across the ArviZ ecosystem (arviz-base, arviz-stats, and arviz-plots). While the recent modular split has helped organize the codebase, it has also led to inconsistencies in CI workflows, testing setups, and type checking. The goal of this project is to make these systems consistent, stricter, and easier to maintain across all repositories. To achieve this, we will unify tools like tox and pre-commit, improve type safety using docstub and strict Mypy checks, strengthen test coverage by treating warnings as errors and adding missing tests, and introduce nightly CI testing against upcoming versions of scientific Python libraries. This will help contributors work more easily across repositories and ensure that ArviZ remains stable and reliable for users over time.