Force cython when building sdist (see 196 instead)

[perlpunk]

Fixes #182

thanks @asottile

Alternatively there's #33 which will only issue a warning

[asottile]

lg2m!

[ingydotnet]

Looks good. Would have helped me not create the bad 4.1 dist.

[nitzmahone]

The warning probably shouldn't be in setup.py for default usage though- otherwise anyone who installs from sdist will get it. Better to have a test to make sure the dist looks like we want.

[asottile]

@nitzmahone this error will only happen for packagers and not for normal users during install. setup.py sdist isn't part of the install pathway (setup.py install / setup.py bdist_wheel)

If you want to convince yourself, check out this branch, build the sdist with python setup.py sdist and then install it in a fresh virtualenv via pip install path/to/pyyaml.tar.gz

[perlpunk]

I think @nitzmahone meant the warning, not the error.
I included it because the comment in #33 suggested that it would be a good idea.
We can remove it if it doesn't make sense.

[asottile]

ah! I should learn to read more carefully :)

yeah the warning probably isn't necessary given we'll have the .c file now 🤷‍♂️

[perlpunk]

I created #196