textproc/py-pystemmer: Fix build with Python 3.7

setup.py contains a conditional checking if it can import cython. If not, the
included (but stale) src/Stemmer.c is used. Add in cython as a build dependency
to force regeneration of the C file from the actual source src/Stemmer.pyx.
Also bump PORTREVISION to reflect possible differences between included and
regenerated src/Stemmer.c.

https://github.com/snowballstem/pystemmer/issues/18

PR: 229629
Submitted by: Charlie Li <ml+freebsd@vishwin.info>
Reported by: Pascal Christen <pascal.christen@hostpoint.ch>