Update test suite to cope with LabThings validating properties on write...

This MR enables validation of properties when they are set from within Python. This requires the next release of LabThings, and sits behind a feature flag.

This MR contains the following

• Use a branch of LabThings (should be changed to the release in pyproject.toml before merge)
• Enable the feature flag in __init__. I would love to come up with a neater place to put this!
• Update relevant tests to check for ValidationError rather than the old behaviour.

Before merge:

• Merge !561 and rebase.
• Update to the next release of LabThings (probably in a separate MR).
• Rebase so this MR is using the same version of LabThings as v3.

Merge checklist:

• [ ] All new/changed functions have up to date typehints and docstrings
• [ ] Any UI updates have a before and after screenshot in the MR description
• [ ] Unit testing of new functionality has similar or better coverage than similar existing code
• This code has been tested on a microscope
  • [ ] Webapp has been recompiled, with aesthetic changes checked in light and dark mode (if changed)
  • [ ] With updated settings JSON (if changed)
  • SmartScan and live stitching work