linting: enable pycodestyle for tests

[jku]

We're currently ignoring pycodestyle errors ruleset ("E") in pyproject.toml

[tool.ruff.lint.per-file-ignores]
"tests/*" = ["D", "E"]
"examples/*/*" = ["D"] 

We should

• not ignore "E" for tests
• fix the issues in tests or add # noqa: XXX comments to silence specific places in code
• if it looks like there is a specific rule we don't want to follow and there are too many violations, we can add that rule to per-file-ignores