Bug#986527: It's a build-dep issue

[Julien Puydt]

Hi,

I had a look:
- sagemath build-depends on cython3
- /usr/bin/cython is in cython
- /usr/bin/cython3 is in cython3

so I see two ways out :

(1) change the b-dep to cython (from cython3) [and cython-dbg from
cython3-dbg, I guess] ;

(2) make so cython is called as cython3.

I can't give it a try before at least a week, so if someone wants to
dive in...

JP