Add CI workflow and contribution guidelines

- GitHub Actions workflow for testing on Python 3.9-3.12
- Ruff linting in CI
- Coverage reporting to Codecov
- CONTRIBUTING.md with development setup instructions
- LICENSE file (MIT)
- Dev dependencies in pyproject.toml

.github/workflows/ci.yml

@@ -0,0 +1,59 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ["3.9", "3.10", "3.11", "3.12"]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install pytest pytest-cov
+          pip install -e .
+
+      - name: Run tests
+        run: |
+          python -m pytest tests/ -v --cov=inventree_rma_plugin --cov-report=xml
+
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v4
+        if: matrix.python-version == '3.11'
+        with:
+          files: ./coverage.xml
+          fail_ci_if_error: false
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Install linters
+        run: |
+          python -m pip install --upgrade pip
+          pip install ruff
+
+      - name: Run ruff
+        run: |
+          ruff check --output-format=github .