Fix: Support Python 3.9 and 3.10 for linting too

Run the linter with Python >= 3.9 as the target. This wasn't an issue
yet but might have caused problems with running ruff with older Python
version ins future. Until the end of the year we still support Python
3.9.

pyproject.toml

@@ -63,7 +63,7 @@ exclude = '''
 
 [tool.ruff]
 line-length = 80
-target-version = "py311"
+target-version = "py39"
 
 [tool.ruff.lint]
 extend-select = ["I", "PLE", "PLW"]