🐛 Fix Python3.11+ raises `ModuleNotFoundError: No module named typing_extensions`

[waketzheng]

How to reproduce:

mkdir temp
cd temp
python3.11 -m venv venv
source venv/*/activate
pip install asyncer
python -c 'import asyncer'

BTW: I changed anyio.sleep(0) to anyio.lowlevel.checkpoint as ruff check --extend-select=ASYNC suggested. And remove the file poetry.toml as no longer need it.