Upgrade to Cython 0.21
Trac #16912: Upgrade Cython to 0.21

Download this file and rename it to
`$SAGE_ROOT/upstream/cython-0.21.tar.gz`:
[http://cython.org/release/Cython-0.21.tar.gz]

This is needed to fix #16113.

URL: http://trac.sagemath.org/16912
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Volker Braun