lint: specify ruff target-version

`ruff` 0.8.0 (released 2024-11-22) no longer defaults to supporting Python 3.8,

> Ruff now defaults to Python 3.9 instead of 3.8 if no explicit Python version
> is configured using [`ruff.target-version`](https://docs.astral.sh/ruff/settings/#target-version)
> or [`project.requires-python`](https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#python-requires)
> ([#13896](astral-sh/ruff#13896))
> — https://github.com/astral-sh/ruff/blob/f3dac27e9aa6ac6a20fc2fb27ff2e4f5d369b076/CHANGELOG.md#080

We want to support Python 3.8 until February 2025, so we need to set
`target-version`.

> The minimum Python version to target, e.g., when considering automatic code
> upgrades, like rewriting type annotations. Ruff will not propose changes
> using features that are not available in the given version.
> — https://docs.astral.sh/ruff/settings/#target-version

Author: joar

python_files/pyproject.toml

@@ -23,6 +23,7 @@ ignore = [
 
 [tool.ruff]
 line-length = 100
+target-version = "py38"
 exclude = [
     "**/.data",
     "lib",