Add linting CI

.gitlab-ci.yml

+image: python:3.10
+
+stages:
+  - dependencies
+  - lint
+
+download_dependencies:
+  stage: dependencies
+  before_script:
+    - venv .venv
+    - source .venv/bin/activate
+  script:
+    - pip install -r requirements.txt
+  artifacts:
+    paths:
+      - .venv
+
+pylint:
+  stage: lint
+  dependencies:
+    - download_dependencies
+  needs:
+    - download_dependencies
+  before_script:
+    - pip install pylint
+  script:
+    - pylint calculator --fail-on=error

calculator/operators.py

@@ -22,3 +22,11 @@ STANDARD_OPERATORS = {
     '*': Operator('×', 2, lambda a, b: a * b),
     '/': Operator('/', 2, lambda a, b: a / b),
 }
+
+def test_operator():
+    """
+    Test the Operator class.
+    """
+    operator = Operator('%', 1, lambda a, b: a % b)
+    assert repr(operator) == '%'
+    assert operator(15, 4) == 3