Skip DeathReaper tests when `pydiscourse` is not importable (#443)

Depending whether Poetry virtualenv contains `pydiscourse`,
it will skip or not `tests.unit.wot.exclusions`

pydiscourse is installed when specifying `poetry install --extras deathreaper`

https://docs.pytest.org/en/7.1.x/how-to/skipping.html#skipping-on-a-missing-import-dependency