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

atlassian-tac
4.0.247

This version was deleted from PyPI around 2023-09-29. Installing fails with:
$ pip3 install atlassian-tac==4.0.247 ERROR: Could not find a version that satisfies the requirement atlassian-tac==4.0.247

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/2023-09-28/ \ atlassian-tac==4.0.247
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Or, download files manually

And install manually via pip: