use modern async backend

**Is your feature request related to a problem? Please describe.**
Currently gdbgui's backend uses SocketIO and has a dependency on gevent, an older async implementation. Additionally, SocketIO is not needed in all websocket implementations.

**Describe the solution you'd like**
Use a modern async backend that does not require gevent/eventlet, nor SocketIO. A good candidate is FastAPI. Another is Starlette, which FastAPI is based on.

https://fastapi.tiangolo.com/
https://www.starlette.io/