Remove cython from install requires

[trxcllnt]

Cython is a build-only dependency, so this PR removes Cython from the install_requires list of the hdbscan package.

setup.py reads requirements.txt to populate its runtime requirements, and requirements.txt incldues cython. So removing it from requirements.txt should be sufficient to fix this.

I can make a separate requirements-build.txt if the maintainers prefer using a requirements file for build dependencies.