[Debian-science-sagemath] Upgrade Cython in Debian / sage 8.2 status

[Ximin Luo]

Tobias Hansen:
> I accidentally uploaded flint-arb 2.13 to unstable and had to introduce an epoch to replace it by version 2.12. Sorry about that Snark...
> 

In future if this happens, I believe the recommended way is to upload a version called 2.13+actually2.12 or something like that.

The justification is that epochs carry a long-term ugliness in the version, so if the mistake is temporary and will get fixed by a soon-to-be-uploaded newer version then it's better to just hack the version.

source: some random conversation on #debian-devel that I forgot the context of

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