Refine Python CI

Add lint environment to `deltachat-rpc-client/`
and set line length to 120, same as in `python/`.

Switch from flake8 to ruff.

Fix ruff warnings.

deltachat-rpc-client/tox.ini

@@ -2,6 +2,7 @@
 isolated_build = true
 envlist =
     py3
+    lint
 
 [testenv]
 commands =
@@ -16,3 +17,13 @@ deps =
     pytest-asyncio
     aiohttp
     aiodns
+
+[testenv:lint]
+skipsdist = True
+skip_install = True
+deps =
+    ruff
+    black
+commands =
+    black --check src/ examples/ tests/
+    ruff src/ examples/ tests/