Update numpy to v1.18.1 (add cython recipe)

[opacam]

We add the cython recipe because the last version of numpy (at time of writing v1.18.1), requires cython installed as a Python module during the build stage of the numpy recipe.

Also this will make the numpy recipe only available for Python3 since it requires python>=3.5.

[opacam]

Seems that the new numpy recipe is also failing for MacOs...similar error as the old numpy version tested at #2046 (see this comment)...anyway I just wanted to test in here, imho, it is not a good idea rely such test on MacOs build, so I will move it.

Note:: Just noting that this cannot break what is already broken, but we still will lose numpy for Python2...well...I don't think that this fact is so important considering that it won't be maintained anymore...we should remove it, probably sooner than later

[AndreMiras]

LGTM