Add nlpo3 to Test README

Author: bact

tests/README.md

@@ -16,7 +16,7 @@ Tests are categorized into three groups: core, compact, and extra.
 - Run `unittest tests.compact`
 - Test a limited set of functionalities that rely on a stable and small subset
   of optional dependencies specified in `requirements.txt`.
-- These dependencies are `PyYAML`, `numpy`, `pyicu`,
+- These dependencies are `PyYAML`, `nlpo3`, `numpy`, `pyicu`,
   `python-crfsuite`, and `requests`.
 - Test with the latest two stable Python versions.