pin jupyter_telemetry dependency

[Zsailer]

Some changes in jupyter_telemetry are pending that will lead to breaking changes here in jupyterhub. jupyter_telemetry is likely going to require event schemas to explicitly state if+when they are collecting personal data. This is critically important for protecting user's personal data. Once these changes land in jupyter_telemetry, JupyterHub's event schemas will need updating to include the new, required fields.

In the meantime, I think it's wise for jupyterhub to pin its dependency to the current release of jupyter_telemetry so we don't have to coordinate a release of jupyter_telemetry and jupyterhub w/ new event schemas.

I'll update JupyterHub's event schemas once jupyter/telemetry#46 merges and jupyter_telemetry has a new release.

[consideRatio]

Thanks for considering this Zach! ❤️