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_remote.py

@@ -10,7 +10,7 @@ import warnings
 from getpass import GetPassWarning
 from pathlib import Path
 from shutil import which
-from typing import Any, Optional
+from typing import Any
 
 import pytest
 
@@ -30,7 +30,7 @@ class FakeSSH:
         self.home = home
         self.bin_dir = bin_dir
         self.commands: list[str] = []
-        self.fail_on: Optional[str] = None
+        self.fail_on: str | None = None
         self.closed = False
 
     def exec_command(self, command: str) -> tuple[None, io.BytesIO, io.BytesIO]:
@@ -186,7 +186,7 @@ class TestRemoveOldBackups:
         assert [c for c in ssh.commands if 'rm -r' in c][0].startswith('sudo rm -r ')
 
     @pytest.mark.parametrize('names', [[], None])
-    def test_nothing_to_remove(self, remote_home: Path, names: Optional[list[str]]) -> None:
+    def test_nothing_to_remove(self, remote_home: Path, names: list[str] | None) -> None:
         if names is not None:
             make_backups(remote_home, *names)
 
@@ -348,11 +348,11 @@ class TestRsyncArgs:
 class FakeClient:
     """paramiko.SSHClient stand-in; connect() takes its outcomes from a list"""
 
-    def __init__(self, outcomes: list[Optional[BaseException]]) -> None:
+    def __init__(self, outcomes: list[BaseException | None]) -> None:
         self.outcomes = outcomes
         self.connects: list[dict[str, Any]] = []
         self.policies: list[str] = []
-        self.known_hosts: Optional[str] = None
+        self.known_hosts: str | None = None
         self.closed = False
 
     def load_host_keys(self, filename: str) -> None:
@@ -370,7 +370,7 @@ class FakeClient:
 
         if isinstance(outcome, UserWarning):
             # Like WarningPolicy for an unknown host key
-            warnings.warn(outcome)
+            warnings.warn(outcome, stacklevel=2)
         elif outcome is not None:
             raise outcome
 
@@ -462,7 +462,7 @@ class TestLogin:
     def test_password_no_terminal(self, client: FakeClient, monkeypatch: pytest.MonkeyPatch,
                                   caplog: pytest.LogCaptureFixture) -> None:
         def no_tty(prompt: str) -> str:
-            warnings.warn('Can not control echo on the terminal.', GetPassWarning)
+            warnings.warn('Can not control echo on the terminal.', GetPassWarning, stacklevel=2)
 
             return ''
 
@@ -474,7 +474,7 @@ class TestLogin:
 
     @pytest.mark.parametrize('outcome, connected', [(None, True), ('error', False)])
     def test_keyfile(self, client: FakeClient, monkeypatch: pytest.MonkeyPatch,
-                     outcome: Optional[str], connected: bool) -> None:
+                     outcome: str | None, connected: bool) -> None:
         pkey = object()
         client.outcomes = [ssh_error(), ssh_error(), ssh_error() if outcome else None]
         monkeypatch.setattr(sb.Backup, '_load_ssh_key', lambda self: pkey)
@@ -548,7 +548,7 @@ class TestLoadKey:
     def test_no_terminal(self, keys: Path, monkeypatch: pytest.MonkeyPatch,
                          caplog: pytest.LogCaptureFixture) -> None:
         def no_tty(prompt: str) -> str:
-            warnings.warn('Can not control echo on the terminal.', GetPassWarning)
+            warnings.warn('Can not control echo on the terminal.', GetPassWarning, stacklevel=2)
 
             return ''
 
@@ -558,7 +558,7 @@ class TestLoadKey:
         assert 'Unable to get passphrase' in caplog.text
 
     @pytest.mark.parametrize('content', [None, 'not a key\n'])
-    def test_bad_file(self, tmp_path: Path, caplog: pytest.LogCaptureFixture, content: Optional[str]) -> None:
+    def test_bad_file(self, tmp_path: Path, caplog: pytest.LogCaptureFixture, content: str | None) -> None:
         keyfile = tmp_path / 'id'
 
         if content is not None: