Support for PEP-592 (yanked releases)

[sdispater]

PyPI and pip recently added support for "yanked" releases (PEP-592).

We should add support for it in Poetry as well.

This should be handled in two places:

• In the PyPiRepository class: the PyPI JSON API provides a yanked property at the release level that we can use to flag a file as yanked.
• In the LegacyRepository class: this one should follow the PEP to retrieve the "yanked" information.

[neersighted]

Completed in 1.2