Package preview feature for PyPI

[xmunoz]

Right now, there are ways for package maintainers to test and share draft versions of their upcoming releases, but they cause friction and confusion. So we want to add staged releases -- a temporary state that a release can be in, where PyPI has it and can evaluate it, but hasn't published it yet.

This will:

• let project owners/maintainers preview/test how their package metadata displays on the website, and review where their fresh releases are out of compliance with site and interoperability requirements (preventing the problem of maintainers wanting to re-upload removed files)
• help cross-platform package maintainers coordinate dozens of wheels built on multiple machines for simultaneous release
• Provide an interoperability check for toolchain developers, and a testing site for people learning packaging
• Simplify packagers' upload configuration files
• reduce complexity that currently forces maintainers to use confusing "dev" or prerelease version numbers
• Improve security of package uploads, by allowing maintainers to scope upload API tokens to the newly staged package
• Prevent package name conflicts
• Streamline infrastructure maintenance and confusing documentation by letting us take down the separate test.pypi.org staging site
• Provide pre-release warnings to maintainers of packages that fail metadata checks (such as rejecting or warning for packages without Python requirements metadata, or manylinux wheels that fail auditwheel checks -- as we increase the packaging ecology's strictness regarding metadata standards compliance, during the intermediate period where we're warning maintainers/owners about failing strictness checks but not yet blocking releases on those new stricter checks, the package preview feature will help us provide soft warnings.

We'll need database support for understanding the release state ("is this published or not"), user experience and developer support, and testing, security, infrastructure, and project management support.

[xmunoz]

Looks like there is already a PR open for this, so maybe not a great candidate for funding: pypi/warehouse#8941