Work around ruff issue

The minimum Python version is Python 3.8. From `pyproject.toml`:
	requires-python = ">=3.8"

However, `ruff.toml` shadows `pyproject.toml`, prompting ruff to use its
own default target, which is currently Python 3.9.

ruff.toml

@@ -1,5 +1,8 @@
 extend = "ruff_defaults.toml"
 
+# https://github.com/astral-sh/ruff/issues/10299
+target-version = "py38"
+
 # https://github.com/astral-sh/ruff/issues/8627
 exclude = [".git", ".mypy_cache", ".ruff_cache", ".venv", "dist"]