Add type checking to standard tests

.github/workflows/ci.yml

@@ -25,6 +25,10 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         python -m pip install .
+    - name: Type Checking
+      run: |
+        python -m pip install mypy
+        python -m mypy iso639
     - name: Test
       run: |
         pip install pytest 

CONTRIBUTING.md

@@ -64,9 +64,10 @@ Pushing changes to your forked repository should trigger a [workflow](https://gi
 flake8
 black --check .
 isort --check-only .
+mypy iso639
 ```
 
-`iso639-lang` uses `flake8` for linting, `black` for formatting and `isort` for sorting imports. The configuration of these tools is available in the `.flake8` and `pyproject.toml` files.
+`iso639-lang` uses `flake8` for linting, `black` for formatting, `isort` for sorting imports and `mypy` for type checking. The configuration of these tools is available in the `.flake8` and `pyproject.toml` files.
 
 
 ## Submitting Changes

requirements-dev.txt

@@ -5,4 +5,5 @@ isort>=5.13.2
 lxml>=5.3.0
 pandas>=2.0.3
 requests>=2.32.3
-pytest>=8.3.2
+pytest>=8.3.2
+mypy>=1.13.0