Client-only installation is broken

[rwb27]

Describe the bug
When we install without the server extra, the module won't import

python -m venv .venv
.venv/Scripts/activate
pip install .

In Python

import labthings_fastapi  # Fails because there's no `fastapi` installed

Expected behaviour
The module should import, though only labthings_fastapi.client is expected to work.

Additional context
This may be the final push we need to split out a proper client package. Advice welcome on how to coexist two Python packages nicely in one repo, or how best to manage the interdependency.

[julianstirling]

I would keep them in the same repo and the same package otherwise synchronisation will be a pain. I would however make a second set of tests (in a separate directory) that only have client code.

This way it is possible to make a docker container that fires up a defined server from one venv, opens a new venv install only the client package, runs tests that talk to that server. If we really do want client only installs then as this is closing with #123 it may be worth making a new issue that is "Write client only tests, before making server dependencies "

However, I think in the short time client only installs aren't overly helpful. The servers are not huge packages to install compared to say numpy, and there isn't a lot of server code out there for there to be a great demand for client only install. From my own experience I tend to skip "how to install" for packages because if a package is on PyPi I assume running pip install package_name will give me the core functionality for the first examples in the docs, so I missed that I needed to install with [server] the first time.