Make poetry setup future proof

[pirmink]

During the Jenkins build, the following warning are issued:

Creating virtualenv pyflexplot in /src/app-root/.venv
Building pyflexplot (2.3.1)
  - Building wheel
  - Built pyflexplot-2.3.1-py3-none-any.whl
Warning: poetry-plugin-export will not be installed by default in a future version of Poetry.
In order to avoid a breaking change and make your automation forward-compatible, please install poetry-plugin-export explicitly. See https://python-poetry.org/docs/plugins/#using-plugins for details on how to install a plugin.
To disable this warning run 'poetry config warnings.export false'.
Warning: poetry-plugin-export will not be installed by default in a future version of Poetry.
In order to avoid a breaking change and make your automation forward-compatible, please install poetry-plugin-export explicitly. See https://python-poetry.org/docs/plugins/#using-plugins for details on how to install a plugin.
To disable this warning run 'poetry config warnings.export false'.
The `--dev` option is deprecated, use the `--with dev` notation instead.
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