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For making space for the latest release I had to clean up versions on PyPI. They allow a total of 10GB space for the project and we exceeded it. For some reason, the binaries are growing pretty large now and there is a large matrix of Python versions / platforms.
Specifically this means, all 0.28.x versions except the latest one (0.28.17) have been removed. I think 0.28.17 is a good representative for the 0.28 branch and I'd suggest to use this version if you do not wish to switch to the latest one.
In case you absolutely need a different version, I can restore them by making a post release. But as this means considerable effort, so I would try to avoid that case. My general recommendation is to use something like
pip install "klayout>=0.28,<0.29"
I want keep the latest minor version of a major release, so that specification should work for a long time.
Matthias