Fix hot reloading for Python 3.9

[MtkN1]

Fixes #178

1. Move asyncio.Event creation to lifespan

This PR moves the creation of the asyncio.Event from RebuildServer.__init__ to RebuildServer.lifespan. This ensures asyncio.Event is created within the event loop context, resolving the relevant error.

Note

The error does not occur in Python>=3.10 because the implementation was changed by python/cpython#86558.

2. Add rebuild test

Since there was no rebuild test, I added one.

• Added a rebuild test.
  • Added a test to check refresh messages from /websocket-reload after modifying the index.rst file.
• Changed the target test function to an asynchronous function.
  • Starlette's TestClient uses threads, while RebuildServer uses multiprocessing. In my view, this affected the rebuild test execution and cause it to fail. (0662fc9)
  • Changed Starlette's TestClient to http.AsyncClient. (ref)
  • Added test dependencies asgi-lifespan and httpx-ws required for testing lifespan and WebSocket.

[MtkN1]

After four months, the CI is finally green! ✅