Let requires-python set the ruff target version

[Danipulok]

What

Delete target-version = "py39" from [tool.ruff], and rewrite the two isinstance calls that the raised target flags.

Why

requires-python is >=3.10, so ruff was linting the package against a floor two releases below the one it declares.

Deleting the key rather than updating it means it cannot drift again: when target-version is absent, ruff reads the floor from requires-python.

The higher target surfaces two UP038 findings in run_code.py. Both are rewritten to the X | Y form, which isinstance has accepted since 3.10.

How to test

make lint, make typecheck and the test suite. No behaviour change.