make cython an install time dependency and do not ship .c files in sdist

[tacaswell]

The .c files generated by cython are not forward compatible so the current sdist will not install on cpython master (and given the errors, I assume 3.8b4, but I have not tried).

Having a cython dependency when installing from source is not too bad (cython can be pip-installed and you need a compiler either way) and wheels can be used to serve users who need to have no install time dependencies.

This is the opposite request of #4.

Installing from the repository works correctly.

[eriknw]

Oh, I see. Thanks for the explanation @tacaswell. It's probably not so bad to had a dependency on Cython. I'll try to get this resolved and published on Python 3.8 in a week or two.

[tacaswell]

Another option is what skimage does. If cython is available, they re-cythonize otherwise fallback to the bundled c.

[eriknw]

That seems pragmatic.

[eriknw]

Thanks for the suggestion @tacaswell.

Please see #137

[eriknw]

Done! Thanks for the insight @tacaswell that Python 3.8 failed due to Cython's *.c files not being forward compatible. (This isn't Cython's fault. Python changed.)

Cython is an optional dependency for pip install cytoolz.

Use pip install 'cytoolz[cython]' to require Cython.

[tacaswell]

Thanks!