Add pytest-icdiff to facilitate visualising differences in failed tests

[ns-rse]

The pytest-icdiff extension to pytest makes visualising and reading the differences in assertions easier.

Developers would benefit from this visual aid so we should include it as a an optional dependency for developers in pyproject.toml.

[project.optional-dependencies]
...
dev = [
  ...
  pytest-icdiff
  ...
]