Cython installed in BEFORE ALL is not seen by setup.py

[abitrolly]

In this job https://github.com/abitrolly/opendht/runs/6950755175?check_suite_focus=true Cython is installed in BEFORE ALL step, but setup.py can not see it. Is this by design?

[Czaki]

yes. Because BEFORE_ALL is executed once on host python. To have option for cython visible in setup.py you need to install it in BEFORE_BUILD Then it will be installed in same python as used for build.

But it may not work if build discover that isolated build is required. Then only option to hace cython is use pyproject.toml and put cython in build-system section in list requires like in this comment #1139 (comment)