[pyupgrade] Functional enums (UP048)

[InSyncWithFoo]

Summary

Resolves #12417.

Test Plan

cargo nextest run and cargo insta test.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+3 -0 violations, +0 -0 fixes in 3 projects; 52 projects unchanged)

apache/airflow (+1 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --no-fix --output-format concise --preview --select ALL

+ tests/serialization/test_serde.py:152:9: UP048 [*] Enum declared using functional syntax

python-trio/trio (+1 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --no-fix --output-format concise --preview

+ src/trio/_ssl.py:253:1: UP048 [*] Enum declared using functional syntax

pytest-dev/pytest (+1 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --no-fix --output-format concise --preview

+ testing/test_compat.py:163:5: UP048 [*] Enum declared using functional syntax

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
UP048	3	3	0	0	0

[MichaReiser]

I'm not sure UP is a good fit. Using class—or functional-based enums seems more a matter of taste than whether one is considered better (because it was intentionally designed for it) than the other.

I think this is also visible in the ecosystem checks, where the function-based notation is more concise than the class one.

Overall, I think we should hold off with this rule. It seems too opinionated.