QA test packages from PyPI without distorting downloads

[Josverl]

• it should be possible to use pip install --user-agent "ignorestats/1.0" some_package
• or to use one of the clients that are already ignored bandersnatch
• or by running https://github.com/JonasAlfredsson/docker-devpi somewhere to cache (but not open on the net)

or just trusts the pip caching locally and in CI