Comment 15 for bug 1833229

This might be a simpler way to reproduce it. I uploaded a package to test.pypi and wanted to use --extra-index-url to verify that its dependencies are pulled in (it has dependencies that are in pypi but not in test.pypi). Here are my steps to reproduce it:

python3 -m venv /tmp/test3env
source /tmp/test3env/bin/activate
pip install wheel
pip install --index-url https://test.pypi.org/simple/ --extra-index-url https://pypi.org/simple pytopo==1.6.1

which gives the error:

Could not install packages due to an EnvironmentError: 404 Client Error: Not Found for url: https://test.pypi.org/simple/pycairo/

Sometimes it errors on a different package, not always pycairo, but the rest of the error is the same. I filed https://github.com/pypa/pip/issues/7486 upstream and I suspect they're right that it's the same issue as this Ubuntu bug (I'm on 19.10).