ci: do not use --encoding option for rst-lint

It was removed in rst-lint 2.0:
twolfson/restructuredtext-lint@7b43036

Author: link2xt

python/tox.ini

@@ -46,7 +46,7 @@ deps =
 commands =
     ruff format --diff setup.py src/deltachat examples/ tests/
     ruff check src/deltachat tests/ examples/
-    rst-lint --encoding 'utf-8' README.rst
+    rst-lint README.rst
 
 [testenv:mypy]
 deps =