Pin run-env cython for consistency with requirements.txt

[matbryan52]

We noted here conda-forge/libertem-blobfinder-feedstock#1 that hdbscan causes pip check to fail as the conda recipe in this repo requires any cython but the requirements.txt file (https://github.com/scikit-learn-contrib/hdbscan/blob/0.8.33/requirements.txt, read into setup.py and used by pip) limits cython to >=0.27,<3. The pin would be temporary until this PR is merged scikit-learn-contrib/hdbscan#624 to remove cython at runtime and the conda recipe also updated.

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[matbryan52]

@conda-forge-admin please rerender