Install extras so that doctest can touch all modules

Author: sweeneytr

.github/workflows/test.yaml

@@ -19,7 +19,7 @@ jobs:
     - name: Install and run tests
       run: |
         curl -sSL https://install.python-poetry.org | python3 -
-        poetry install
+        poetry install -E cli
         poetry run pytest --cov=fast_json_pointer --cov-report lcov tests/ --doctest-modules src/
 
     - name: Coveralls