To everybody who mentioned that Cython sources don't belong on sys.path: where *do* they belong then?

One issue which hasn't been mentioned before is packaging. By installing Cython sources along with the generated .so files, we can re-use all the existing tooling from distutils/setuptools/wheel to install those source files. If we want to install them in a different way, ideally it should be done in a way sanctioned by the PyPA and the tools should support it. We should then also change linecache to search for source files in that directory.

Unless somebody can come up with a simple suggestion, this seems to require basically a PEP. I don't plan to write that PEP and fight for it, since this can easily be fixed with essentially a 1-line patch to linecache, as I proposed in https://github.com/python/cpython/pull/6653