Avoid installing the latest MLfow to prevent doctests from failing #3135
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Motivation
Although I haven't investigated the cause yet, the doctest was failing with the latest MLflow.
https://github.com/optuna/optuna/runs/4368455449?check_suite_focus=true
I confirmed the problem doesn't occur with MLflow v1.21.0.
https://github.com/optuna/optuna/runs/4368582442?check_suite_focus=true
So, as a tentative patch, I introduce the version constraint.
Description of the changes