Add dill to test dependencies

.github/workflows/run_tests.yml

@@ -26,7 +26,7 @@ jobs:
           python -m pip install --upgrade pip
           pip install pytest mypy codecov pytest-cov pandas
           pip install plotnine matplotlib scipy scikit-learn
-          pip install lightgbm pyarrow black isort
+          pip install lightgbm pyarrow black isort dill
 
       - name: MyPy Checks
         run: mypy miceforest --ignore-missing-imports