Wrong Python version causing Attribute Error is_integer() in utils.py

https://github.com/google/tunix/blob/3b558520bb65e4cd649f8199362f9dc144e541da/pyproject.toml#L8

This version should be set to '>=3.12' (or higher) at least, because tunix/tunix/rl/utils.py calls value.is_integer() which is safely supported for the base `int` type since Python v3.12. For example, see below:
https://github.com/google/tunix/blob/3b558520bb65e4cd649f8199362f9dc144e541da/tunix/rl/utils.py#L37

I tested it with v3.11 and confirmed it causes Attribute Error. You should upgrade the default Python version to 3.12 (or higher) in pyproject.toml or substitute all value.is_integer() with the more primitive and safe isinstance().