I have tried uploading a new project using twine without a repository set in my .pypirc and with https://upload.pypi.org/ set in my .pypirc.

By default (ie with no repository set) twine 1.9.1 appears to attempt to upload to the legacy url https://upload.pypi.org/legacy/ but I get the error:
HTTPError: 400 Client Error: home_page: Invalid URI for url: https://upload.pypi.org/legacy/

By setting it to what I think is the correct url I get the error:
HTTPError: 405 Client Error: Method Not Allowed for url: https://upload.pypi.org/

I am using pip 9.0.1, twine 1.9.1, setuptools 38.4.0