You're looking at StableBuild's Deleted PyPI mirror - where we index all deleted packages from the Python package registry.

arts
2023.9.20

This version was deleted from PyPI around 2023-10-29. Installing fails with:
$ pip3 install arts==2023.9.20 ERROR: Could not find a version that satisfies the requirement arts==2023.9.20
View other deleted versions of arts

Install through StableBuild

You can install this package from StableBuild's PyPI snapshots from before the version was deleted:
pip3 install \ -i https://YOUR_API_KEY.pypimirror.stablebuild.com/2023-10-28/ \ arts==2023.9.20
Sign up for StableBuild to get an API key (it's free and takes <60 seconds!)

Or, download files manually

And install manually via pip:
arts-2023.9.20-py2.py3-none-any.whl (208.8KB)
arts-2023.9.20.tar.gz (134.1KB)