Helpful error message when failing to upload to an inaccurate PyPI URL

[brainwane]

#289 (comment) and #381 make me think of this enhancement idea:

If Twine fails to upload with an "HTTPError: 405 Client Error: Method Not Allowed for url", and the URL has "pypi.org/" in it (thus including both pypi.org and test.pypi.org), we'd give the user a helpful error message saying: "You probably want one of these two URLs ([link] and [link])" and mentioning https://warehouse.readthedocs.io/api-reference/legacy/#upload-api .