os.setsid() potentially fails in adapter/__main__.py if attaching to session leader remotely

[fecjanky]

Environment data

• debugpy version: 1.6.3
• OS and version: CentOS 8
• Python version (& distribution if applicable, e.g. Anaconda): python3.9
• Using VS Code or Visual Studio: vscode

Actual behavior

Debugger client connection times out

Expected behavior

Debugger client can successfully attach to injected debugpy server

Steps to reproduce:

1. launch a python program with setsid python ....
2. Attach to it using a remote configuration from vscode, check stderr when attaching to see the actual error message