I may be missing something, but if the core mission for Twine is publishing on PyPI, and twine register (per pypi/warehouse#1267) is a no-op, I think we should deprecate and then remove the user-invocable register command.

I propose we deprecate it in the upcoming version (1.10) and then remove it in 1.11.0.

So, in 1.10.0:

• mark Repository.register() and twine/commands/register.py as deprecated with @deprecated decorators
• add a ..deprecated:: marker to the register guidance in the README

In 1.11.0:

• remove Repository.register() and twine/commands/register.py
• remove register from the list of commands in setup.py
• remove the PackageNotFound exception from twine/exceptions.py
• update the README, the architectural overview in docs/contributing.rst, and the changelog