update dependencies, support Python3.9

[cs01]

• I have added an entry to docs/changelog.md

Summary of changes

The flask-socketio dependency version being used is not compatible with some newer versions of Python. The gevent and eventlet dependencies have consistently been sources of difficulty, and since they are only needed for servers with a large number of connected clients, they aren't really even needed 1, 2. so they have been removed entirely. I'd rather not use socketio at all, but the only websocket support I've seen in Flask uses flask-socketio and gevent. With FastAPI, you can just use websockets and asyncio directly. Even better, replacing Python with node you can use websockets, async/await, AND have pty support on Windows. Maybe someday...

Anyway, in this PR:

• Use requirements.in/requirements.txt, and update dependencies for Python and Node
• Remove dependency on gevent and eventlet
• Officially support only Python 3.9, update CI tests and executable builds accordingly
• Disable pagination in gdb tty by default
• Increment version to 0.15

Test plan

Tested by running

> nox -s build_executable_linux
> ./executable/linux/gdbgui_0.15.0.0 

fixes #359

[cs01]

fixes #382