Getting Unhandled exception in event loop: Press Enter to continue

This appears to be due to cpr events getting lost along the way. This little bit of code reproduces the issue under linux (python 3.7.3, prompt_toolkit 3.0.5):

import os
from asyncio import sleep, get_running_loop, run
from prompt_toolkit.patch_stdout import patch_stdout
from prompt_toolkit.shortcuts import PromptSession

from prompt_toolkit import __version__
print("Version:", __version__)


def _exception_handler(loop, context):
    # Should never get here - exit quickly with error
    os._exit(4)


async def t1():
    'Print stuff to screen quickly so prompt_toolkit needs cpr'
    counter = 0 
    while True:
        print("Counter:", counter)
        counter += 1 
        await sleep(.01)


async def main():
    loop = get_running_loop()
    loop.set_exception_handler(_exception_handler)
    loop.create_task(t1())
    session = PromptSession('% ')
    with patch_stdout():
        await session.prompt_async(set_exception_handler=False)

run(main())

This appears to be due to a cpr event getting lost along the way and then the wait_for_responses task eventually gets reclaimed and we get the error. I'll submit a pr for discussion.

Start by running the provided reproducer under Linux with Python 3.7.3, prompt_toolkit 3.0.5, and the asyncio exception handler. Trace prompt_async, CPR event handling, and the wait_for_responses task to determine how a response is lost. Done means the reproducer no longer reaches the unhandled-exception handler while output is being printed.