Marking dependencies as optional, and removing redundant ones

When `notebook` is added as a dependency in [poetry](https://python-poetry.org/), it significantly increases the runtime of a `poetry update`, from ~5s to ~120s, due to the many required dependencies.
I noticed that a couple of these could redesignated as optional; they aren't part of core functionality, and are used in only one or two places:

- `prometheus-client`
- `argon2-cffi`
- `send2trash`

Furthermore, it appears that `pyzmq` could be removed altogether, because [this reference to an old Tornado 5 issue](https://github.com/jupyter/notebook/blob/160c27d3c23dafe8b42240571db21b0d5cbae2fe/setup.py#L114-L116) should not be relevant anymore.

Start with the dependency declarations in setup.py and locate the usage sites for prometheus-client, argon2-cffi, send2trash, and pyzmq. Check that optional dependencies remain available where used, pyzmq is unnecessary, and dependency resolution improves without breaking core functionality.

Written by the indexing model from the issue text.