Python 3.12 support

[Lilly-May]

PR Checklist

• [X] This comment contains a description of changes (with reason)
• [X] Referenced issue is linked (closes #786)

Description of changes

• Updated CI tests to Python 3.12
• Added graphviz as a dependency for the docs since it's required in the causal inference notebook

[Zethson]

https://github.com/py-why/dowhy/releases/tag/v0.12 & lifelines also supports it now.