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

pyproject.toml

@@ -42,6 +42,35 @@ include = ["inventree_rma_plugin*"]
     "migrations/*",
 ]
 
+[project.optional-dependencies]
+dev = [
+    "pytest",
+    "pytest-cov",
+    "ruff",
+]
+
 [tool.pytest.ini_options]
 testpaths = ["tests"]
 python_files = "test_*.py"
+
+[tool.ruff]
+line-length = 120
+target-version = "py39"
+
+[tool.ruff.lint]
+select = [
+    "E",      # pycodestyle errors
+    "W",      # pycodestyle warnings
+    "F",      # pyflakes
+    "I",      # isort
+    "B",      # flake8-bugbear
+    "C4",     # flake8-comprehensions
+    "UP",     # pyupgrade
+]
+ignore = [
+    "E501",   # line too long (handled by formatter)
+    "B008",   # do not perform function calls in argument defaults
+]
+
+[tool.ruff.lint.isort]
+known-first-party = ["inventree_rma_plugin"]