Fix `__breakpointhook__` when it's called from another thread.

Calling `breakpoint()` in a thread other than the one running the Application
would crash the process with a "RuntimeError: no running event loop".

```
  File ".../site-packages/prompt_toolkit/application/application.py", line 1026, in trace_dispatch
    with app.input.detach():
  File ".../lib/python3.9/contextlib.py", line 119, in __enter__
    return next(self.gen)
  File ".../site-packages/prompt_toolkit/input/vt100.py", line 203, in _detached_input
    loop = get_running_loop()
RuntimeError: no running event loop
```