As a new Twine maintainer I've been running into questions like:

* Now that Warehouse doesn't use "register" anymore, can we deprecate it from distutils, setuptools, and twine? Are any other package indexes or upload tools using it? https://github.com/pypa/twine/issues/311
* It would be nice if Twine could depend on a package index providing an HTTP 201 response in response to a successful upload, and fail on 200 (a response some non-package-index servers will give to an arbitrary POST request).

I do not see specifications to guide me here, e.g., in the official guidance on hosting one's own package index https://packaging.python.org/guides/hosting-your-own-index/ . PEP 301 was long enough ago that it's due an update, and PEP 503 only concerns browsing and download, not upload.

I suggest that I write a PEP specifying an API for uploading to a Python package index. This PEP would partially supersede PEP 301 and would document the Warehouse reference implementation. I would write it in collaboration with the Warehouse maintainers who will develop the reference implementation per pypa/warehouse/issues/284 and maybe add a header referring to compliance with this new standard. And I would consult with the maintainers of packaging and distribution tools such as zest.releaser, flit, poetry, devpi, pypiserver, etc.

Per Nick Coghlan's formulation, my specific goal here would be close to:

> Documenting what the current upload API between twine & warehouse actually is, similar to the way PEP 503 focused on describing the status quo, without making any changes to it. That way, other servers (like devpi) and other upload clients have the info they need to help ensure interoperability.

Since Warehouse is trying to redo its various APIs in the next several months, I think it might be more useful to document and work with the new upload API, but I'm open to feedback on this.