Add CI linting steps

.github/workflows/ci.yml

@@ -0,0 +1,19 @@
+name: CI
+on: [push]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: actions/setup-python@v5
+      with:
+        python-version: '3.10'
+    - name: Install dev requirements
+      run: pip install -r requirements_dev.txt
+    - name: Check black
+      run: black --check iati_sphinx_theme
+    - name: Check isort
+      run: isort --check-only iati_sphinx_theme
+    - name: Check flake8
+      run: flake8 iati_sphinx_theme