Remove unused imports in various python files

There's several python files with unused imports, such as `task_runner.py` not needing the `islice` or `numpy` imports: https://github.com/DiamondLightSource/httomo/blob/25ef9aa6c2ef541ba5ea3a6c3bad96bff9c3145c/httomo/runner/task_runner.py#L1-L30

or `utils.py` not needing the `contextmanager` or `Callable` imports: https://github.com/DiamondLightSource/httomo/blob/25ef9aa6c2ef541ba5ea3a6c3bad96bff9c3145c/httomo/utils.py#L1-L6

It would be good at some point to do a check of all python files and update their imports accordingly, to tidy things up.