Add cython to requirements

[pievalentin]

Without this package the image wont build, it fixed my issue. So it might be worth sharing

[coveralls]

Coverage increased (+0.07%) to 65.436% when pulling e18d591 on focom:patch-1 into 006b376 on Zulko:master.

[tburrows13]

@jonnekaunisto I wonder what you think of this one? It has 2 thumbs up so I guess it is not just a one-off fix. Is it possible that a given docker image can work for some people but not others? (I've never used it).

[jonnekaunisto]

I am not entirely sure why it didn't work for them without Cython. The same docker image should work exactly the same on every machine, but the Dockerfile that is in moviepy right now doesn't have a tagged version for Python or Debian such as what #1175 had in its first commit.

Basically every time you build the image it will get the latest Python and Debian version, which will keep changing as new versions come out.

So the CPython might have been needed in 2018, with the latest release of Debian and Python then, but it doesn't seem to be needed anymore.

It might be good to tag the Dockerfile with some Python and Debian version so that it doesn't break when a new version of either of those comes out.

Or maybe we can write a test that runs the tests in the Docker image periodically so that when it does break we can fix it.

[tburrows13]

Ok, I'll close this, and perhaps at some point you could check that it works on the most recent version of Python and Debian and create a PR with those as tags.

Edit:
I've created a dockerhub built here that I think will get updated whenever my fork is updated. Ideally I suppose it would be connected to Zulko's main branch but that isn't particularly important for now. As far as I can understand, it will alert me when the build fails.