ci: add mypy

.github/workflows/tests.yml

@@ -38,4 +38,22 @@ jobs:
         continue-on-error: true
 
       - name: Run tests
-        run: pytest -v
+        run: pytest -v
+
+  typecheck:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      # Mypy runs on a current supported interpreter while checking code that
+      # remains compatible with BOOTH's minimum Python version, 3.9.
+      - name: Set up Python 3.12
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install boothpy (editable, with dev deps)
+        run: pip install -e ".[dev]"
+
+      - name: Run mypy
+        run: mypy src/booth/

pyproject.toml

@@ -29,7 +29,7 @@ classifiers = [
 dependencies = []
 
 [project.optional-dependencies]
-dev = ["pytest>=7.0"]
+dev = ["pytest>=7.0", "mypy>=1.19,<2.0"]
 
 [project.urls]
 Homepage = "https://github.com/Vedantgitbot/booth"
@@ -45,4 +45,9 @@ where = ["src"]
 booth = ["py.typed"]
 
 [tool.pytest.ini_options]
-testpaths = ["tests"]
+testpaths = ["tests"]
+
+[tool.mypy]
+# Run mypy on Python 3.12 in CI while checking compatibility with BOOTH's
+# minimum supported Python version.
+python_version = "3.9"