Change mypy from release to developement depmendency

[josteinl]

I see that mypy is listed as a dependency in the pyproject.toml

[tool.poetry.dependencies]
mypy = "^1.4.1"  # <-- Suggest moving to dev.dependencies

[tool.poetry.group.dev.dependencies]
mypy = "*"  # <-- Suggest moving mypy here

Also, suggest moving everything from the section [tool.poetry.dev-dependencies] to [tool.poetry.group.dev.dependencies].

If you want a PR, please let me know.

I have not checked if you actually use mypy in your code.

[folt]

Great point, feel free to contribute