ci: add Gitea Actions workflow for tests and linters

- .gitea/workflows/checks.yaml: runs on push to main and every PR,
  matrix Python 3.10-3.13 (matches requires-python >=3.10), steps:
  isort, black, ruff check, ruff format, mypy, pytest
- dev extras gain black, isort (needed by pip install -e .[dev] in CI)
  and setuptools>=61.0 (test_formatters_found_by_setuptools imports
  find_packages at runtime; Python 3.12+ venvs no longer bundle it)
- README gains pipeline status badge

Closes #53

pyproject.toml

@@ -33,6 +33,9 @@ dev = [
     "pytest>=7.0",
     "mypy>=1.0",
     "ruff>=0.1.0",
+    "black>=24.0",
+    "isort>=5.0",
+    "setuptools>=61.0",
 ]
 
 [project.scripts]