🔖 release version 0.8.9 (#66)

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<details open="true"><summary>Generated summary (powered by <a href="https://app.graphite.dev">Graphite</a>)</summary>

> ## TL;DR
> This pull request updates the version of the Python Rebalancer (pyrb) from 0.8.8 to 0.8.9 in the pyproject.toml file.
> 
> ## What changed
> The version of the pyrb in the pyproject.toml file was updated. The diff of the change is as follows:
> 
> ```diff
>  [tool.poetry]
>  name = "pyrb"
> -version = "0.8.8"
> +version = "0.8.9"
>  description = "Python Rebalancer"
>  authors = ["Minki Kim <[email protected]>"]
>  readme = "README.md"
> ```
> 
> ## How to test
> To test this change, you can pull the changes from this branch and check the version of the pyrb in the pyproject.toml file. It should be updated to 0.8.9.
> 
> ## Why make this change
> This change was made to update the version of the pyrb. This is important because it helps to keep track of the different versions of the project and the changes made in each version. It also helps users and contributors to know which version of the project they are using or contributing to.
</details>

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "pyrb"
-version = "0.8.8"
+version = "0.8.9"
 description = "Python Rebalancer"
 authors = ["Minki Kim <[email protected]>"]
 readme = "README.md"