No module named 'yaml' on Microscope server 2.5.0 beta 1

Bug report

Summary

From a clean install of the OS, and running the command to get onto the 2.5 beta, I get ModuleNotFoundError: No module named 'YAML'

Configuration

I'm using: Microscope server 2.5.0 beta 1

Steps to reproduce

Clean flash of Raspbian-OpenFlexure OS
sudo ofm activate
sudo ofm upgrade -u https://build.openflexure.org/openflexure-microscope-server/openflexure-microscope-server-v2.5.0-beta.1.tar.gz
sudo ofm serve

Relevant logs and/or screenshots

(.venv) openflexure-ws@microscope:/var/openflexure $ sudo ofm serve Stopping OFM service... OFM service stopped. Launching server in debug mode Traceback (most recent call last): File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main "__main__", mod_spec) File "/usr/lib/python3.7/runpy.py", line 85, in _run_code exec(code, run_globals) File "/var/openflexure/application/openflexure-microscope-server/openflexure_microscope/api/app.py", line 30, in <module> from openflexure_microscope.config import JSONEncoder File "/var/openflexure/application/openflexure-microscope-server/openflexure_microscope/config.py", line 11, in <module> from labthings.json import LabThingsJSONEncoder File "/var/openflexure/application/openflexure-microscope-server/.venv/src/labthings/src/labthings/__init__.py", line 21, in <module> from .labthing import LabThing File "/var/openflexure/application/openflexure-microscope-server/.venv/src/labthings/src/labthings/labthing.py", line 6, in <module> from apispec_webframeworks.flask import FlaskPlugin File "/var/openflexure/application/openflexure-microscope-server/.venv/lib/python3.7/site-packages/apispec_webframeworks/flask.py", line 74, in <module> from apispec import BasePlugin, yaml_utils File "/var/openflexure/application/openflexure-microscope-server/.venv/lib/python3.7/site-packages/apispec/yaml_utils.py", line 4, in <module> import yaml ModuleNotFoundError: No module named 'yaml'

Additional details

(Anything else you think might be relevant to mention)

/cc @jtc42