python 3.12.9 hangs on Flask

[Masamichi Hosoda]

Hi.

I've found python 3.12.9 hangs on Flask.
After starting the Flask HTTP server,
the first HTTP access works fine but hangs immediately afterward.
Once hung, CTRL+C and sending any signals have no effect.

Here's a reproduction procedure.

```
$ uname -srvmo
CYGWIN_NT-10.0-19045 3.5.7-1.x86_64 2025-01-29 19:46 UTC x86_64 Cygwin
$ python3.12 -m venv --system-site-packages --without-pip .venv
$ source .venv/bin/activate
(.venv) $ python -V
Python 3.12.9
(.venv) $ python -m pip install Flask
[...snip...]
Requirement already satisfied: Jinja2>=3.1.2 in /usr/lib/python3.12/site-packages (from Flask) (3.1.5)
[...snip...]
Requirement already satisfied: MarkupSafe>=2.0 in /usr/lib/python3.12/site-packages (from Jinja2>=3.1.2->Flask) (3.0.2)
[...snip...]
Installing collected packages: Werkzeug, itsdangerous, click, blinker, Flask
Successfully installed Flask-3.1.0 Werkzeug-3.1.3 blinker-1.9.0 click-8.1.8 itsdangerous-2.2.0
(.venv) $ cat app.py
from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello_world() -> str:
    return "<p>Hello, World!</p>"
(.venv) $ flask run
 * Debug mode: off
WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.
 * Running on http://127.0.0.1:5000
Press CTRL+C to quit
127.0.0.1 - - [09/Mar/2025 13:37:32] "GET / HTTP/1.1" 200 -
```

Thanks.