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/test_local.py

@@ -2,9 +2,10 @@
 
 import io
 import os
+from collections.abc import Callable, Iterator
 from datetime import datetime, timedelta
 from pathlib import Path
-from typing import Any, Callable, Iterator, Optional
+from typing import Any
 
 import pytest
 
@@ -52,7 +53,7 @@ def out(tmp_path: Path) -> Path:
     return out
 
 
-def make_backup(out: Path, inputs: list[str], keep: int = -1, exclude: Optional[list[str]] = None,
+def make_backup(out: Path, inputs: list[str], keep: int = -1, exclude: list[str] | None = None,
                 remove_before: bool = False, progress: bool = False) -> sb.Backup:
     backup = sb.Backup(inputs, str(out), exclude or [], keep, OPTIONS, remove_before=remove_before,
                        progress=progress)