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
• Fixed data retrieval for the causal effect estimator in ep.pl.causal_effect(estimate). This change is necessary because dowhy currently does not support Python 3.12 (see here). The most recent version of dowhy compatible with Python 3.12 is 0.8, which stores the _data attribute in the estimator instead of the estimate. We can remove the conditional data retrieval as soon as dowhy supports Python 3.12.