Speed up functional tests by relying on pure Python projects

[mkniewallner]

For functional tests, we rely on real-world packages that we retrieve from PyPI. Some of those packages are non-pure Python projects, and, while most of them provide wheels compatible with recent Python versions, it is not always the case, especially when we want to start testing against a Python version that is yet to be released. This means that we either spend more time installing packages in functional tests, or cannot install some projects at all, as they require system dependencies.

By switching to pure-Python packages, adding support for a new Python version should be more straightforward.