[Bug-gsl] Update information for Python wrapper

[David Cortes]

In the GNU page for GSL: https://www.gnu.org/software/gsl/
the section for wrappers suggests PyrexGsl as a Pyrex wrapper. However,
that package hasn't been updated since a long time (same for pyrex) and
doesn't cover newer additions to GSL. It would be more appropriate to
point towards its fork CythonGSL (https://github.com/twiecki/CythonGSL)
, which works with cython (successor of pyrex).