Pip: Add support for yanked files from a repository (PEP 592)

[brainwane]

What's the problem this feature will solve?
To quote PEP 592:

This PEP proposes adding the ability to mark a particular file download on a simple repository as "yanked". Yanking a file allows authors to effectively delete a file, without breaking things for people who have pinned to exactly a specific version.

Describe the solution you'd like
Have pip respect yanking in the PyPI/Warehouse API.

Additional context

Support for PEP 592 -- Adding "Yank" Support to the Simple API will affect which packages get chosen. This will therefore affect the pip resolver, but does not have to wait for the new resolver rewrite (#988 @pradyunsg).

@dstufft is currently working on the PyPI side in https://github.com/pypa/warehouse/pull/5838 . There's an opportunity for someone to help by working on pip support; not sure whether Donald's working on it and/or whether this feature is blocked on the Warehouse API getting into better shape.

[cjerdonek]

I think it might be better to title this "Support yank ..." or "Add support for yanking ...." Currently, it reads like it's to stop supporting pip in some setting (at least that's how I read it, and didn't know it was about the "yank" feature until clicking through).

[cjerdonek]

I posted PR #6647 for this.