[flake8-async] Make ASYNC251 example error out-of-the-box

[MeGaGiGaGon]

Summary

Part of #18972

This PR makes blocking-sleep-in-async-function (ASYNC251)'s example error out-of-the-box

Old example

async def fetch():
    time.sleep(1)

New example

import time


async def fetch():
    time.sleep(1)

Imports were also added to the Use instead: section to make it valid code out-of-the-box.

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.