[ENH] Improve Cython install speed from source about cython

[scoder]

Well, for these cases, the fastest way to install Cython is (and probably will always be) as a pure Python package, without compiled modules.

[scoder]

Yeah, it's sad that this option isn't automatic in setuptools, and also can't be set via environment variables, pip option, etc.

That said – almost no-one needs to install Cython from sources. There are a few CI setups in third-party projects that do this, but almost everyone should be happy with the binary and Python wheels that we provide for releases.

Is there a reason why you do this often enough to consider it annoying?

[YoSTEALTH]

This would only be for developers that want to try out latest Cython that doesn't have pre-built wheel yet. Like people that submit PR or want to test out the new bug fix...

[YoSTEALTH]

O, you mean NO_CYTHON_COMPILE=true python3 -m pip install --upgrade git+https://github.com/cython/cython/ that is interesting. I didn't think you went to such extreme to make pure python version as well. That works as well.