fix: add dynamic version to pyproject.toml for PEP 621 and uv compliance

[tom21100227]

closes #663

When using uv to install label-studio-ml-backends, it would give an error:

$ git clone https://github.com/HumanSignal/label-studio-ml-backend.git
$ cd label-studio-ml-backend/
$ uv venv # create an uv env
$ uv pip install -e .
...error message...
`pyproject.toml` is using the `[project]` table, but the required
  `project.version` field is neither set nor present in the
  `project.dynamic` list

With the addition in this PR it

1. Ensures PEP compliance
2. People can use uv with label-studio-ml-backends.