Force the use of Cython's old build_ext command

[saimn]

A new build_ext command was added in Cython 0.25 (cython/cython#1456) but it breaks Astropy (I could not find why, see astropy/astropy#5437).

Cython 0.25.1 was released to revert this change:
cython/cython@4ecdd3e

This commit ensures that we always use the old build_ext, whatever the Cython version is. At least until we find why the new build_ext command does not work.

[eteq]

Thanks @saimn... This seemed fine to me by eye, but the tests are failing in a variety of convoluted ways... any idea why?

[saimn]

@eteq - No idea ... Locally tests pass on Py2, and on Py3 I had to use PYTHONDONTWRITEBYTECODE=1 to avoid "sandbox violation" errors with the __pycache__ directories, but I also get these errors on master.

[pllim]

That pyx failure is no more. So is this still relevant?

[MSeifert04]

@pllim I think so, because if I understand it correctly they just reverted to issue a deprecation warning:

C:\...\lib\site-packages\Cython\Distutils\old_build_ext.py:30: 
  UserWarning: Cython.Distutils.old_build_ext does not properly handle dependencies and is deprecated.
  "Cython.Distutils.old_build_ext does not properly handle dependencies "

[saimn]

@pllim - I think it is, as using the new build_ext breaks Astropy it is better to make sure that we always use the old one (until we find what is not working with the new).
However I have no idea why tests are failing ...

[saimn]

Re-running the tests to make sure it was not something broken on Travis/Conda side.

[saimn]

And Travis now passes 😮
Appveyor fails because it tries to run tests for ci-helpers (ERROR collecting ci-helpers/test_env.py). Will fix that in appveyor.yml

[bsipocz]

I think this will need a changelog entry. Could you add one @saimn, and then I can merge?

[saimn]

@bsipocz - Rebased and added changelog.

[eteq]

Looks good to me too. Will merge!