[flake8-async] Make ASYNC100 example error out-of-the-box

[MeGaGiGaGon]

Summary

Part of #18972

This PR makes cancel-scope-no-checkpoint (ASYNC100)'s example error out-of-the-box

Old example

async def func():
    async with asyncio.timeout(2):
        do_something()

New example

import asyncio


async def func():
    async with asyncio.timeout(2):
        do_something()

Test Plan

N/A, no functionality/tests affected

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.