Implement robust interoperability testing across packaging toolchain

[xmunoz]

Ref: https://github.com/pypa/integration-test

We need funding to ensure core packaging tools work well with each other; currently they aren't seamlessly interoperable. This will help us get faster at testing and rolling out bugfixes and features for all Python packaging and distribution tools: well-known projects like pip, virtualenv, and wheel, but also all the downstream projects that depend on them.