Add ruff linting

Configure ruff in pyproject.toml and apply its fixes: modern annotations
(X | None, builtin generics, collections.abc), sorted imports, explicit
stacklevel in test warnings.

tests/conftest.py

@@ -1,7 +1,7 @@
 """Shared fixtures for the simple_backup tests"""
 
 import sys
-from typing import Callable
+from collections.abc import Callable
 
 import pytest