[Cython] Migration of tickets from trac to github

[Jeroen Demeyer]

On 2016-07-29 06:50, Robert Bradshaw wrote:
> I'm not quite sure how to serve the mappings

I think it's really important to keep redirects from the old Trac 
tickets to GitHub. There are a lot of links from SageMath to some Cython 
Trac ticket and it would be a shame to lose that.

>> Related to that: converted ticket #73 has a reference to another trac
>> ticket (#51) in a comment, which now links to the (wrong) github ticket #51.
>> https://github.com/robertwb/issues-import-test/issues/73
>
> Hmm... I should spell that out.

Just make the link explicit to trac.cython.org...