devel/py-pyyaml: fix dependency on cython

pyyaml only builds with lang/cython and doesn't build with lang/cython3,
however USE_PYTHON=cython introduces a build-dependency on cython
executable which is installed by either cython version, thus the
dependency may be fulfilled by cython3 if it's installed, which is
incorrect and leads to a build failure. Fix this by switching to
explicit dependency on cython (and not cython3) package.

Approved by: portmgr blanket