Changing the build toolchain/requirements in such a significant way does seem like a major version break, as it could break downstream consumers attempting to install the package.
Because the build toolchain is "visible", that is, pip isn't just downloading a prebuilt binary every time, I think breaking changes that could cause CI systems or user installs to fail is part of the API contract. Think of what major distributions or software packages do when they want to deprecate support for certain platforms - those are major bumps that typically only occur on incrementing the most significant component of the version.
Hypothetical: suppose the the authors changed setup.py so that it only built on Red Hat Enterprise Linux(tm) version 6. Again, they could do that, it wouldn't change the runtime API. And on all other distributions or installers, it would error.
Would that be a major semver change? Of course it would be. The API contract has to include everything from packaging to use.
Because the build toolchain is "visible", that is, pip isn't just downloading a prebuilt binary every time, I think breaking changes that could cause CI systems or user installs to fail is part of the API contract. Think of what major distributions or software packages do when they want to deprecate support for certain platforms - those are major bumps that typically only occur on incrementing the most significant component of the version.
Hypothetical: suppose the the authors changed setup.py so that it only built on Red Hat Enterprise Linux(tm) version 6. Again, they could do that, it wouldn't change the runtime API. And on all other distributions or installers, it would error.
Would that be a major semver change? Of course it would be. The API contract has to include everything from packaging to use.