Add stub setup.py for editable installations.

[ntouran]

In order to run:

pip install -e .[docs,tests]

We found that you either need to add a stub setup.py like this or be using a very recent version of pip. In order to streamline the installation process on more machines with a relaxed pip version constraint, this stub is added.

This fix is recommended by https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html

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Checklist

• [x] This PR has only one purpose or idea.
• [x] Tests have been added/updated to verify that the new/changed code works.

• [x] The code style follows good practices.
• [x] The commit message(s) follow good practices.

• [x] The release notes (location doc/release/0.X.rst) are up-to-date with any important changes.
• [x] The documentation is still up-to-date in the doc folder.
• [x] If any requirements were affected, mention it in the release notes.
• [x] The dependencies are still up-to-date in pyproject.toml.

[john-science]

I'm not ignoring this, I am just very busy with an approaching deadline.

[john-science]

I have opened an alternate PR to this here: https://github.com/terrapower/armi/pull/1664