Cython: upgrade to 0.20.1
Fix various declarations
Fix version and checksum for Cython 0.20.1
Trac #15755: Upgrade cython to version 0.20.1

Upstream tarball: [http://cython.org/release/Cython-0.20.1.tar.gz]

URL: http://trac.sagemath.org/15755
Reported by: ohanar
Ticket author(s): R. Andrew Ohana, Jeroen Demeyer
Reviewer(s): R. Andrew Ohana