usayncio: TypeError: exceptions must derive from BaseException

I have written some code that uses the asyncio module. The entrypoint point to my code looks like this:

try:
    control.main()
except KeyboardInterrupt:
    pass
except Exception as exc:
    sys.print_exception(exc)
    print('* resetting in 10 seconds')
    time.sleep(10)
    machine.reset()

And control.main looks like:

def main():
    [...]

    try:
        loop.run_forever()
        loop.close()
    finally:
        for task in tasklist:
            task.close()

I just upgraded to the version in micropython-lib as of 9e21d6e, and now when I interrupt my code with ^C I see the following error:

Traceback (most recent call last):
  File "snakecharmer/main.py", line 10, in <module>
  File "snakecharmer/control.py", line 52, in main
  File "uasyncio/__init__.py", line 246, in start_server
TypeError: exceptions must derive from BaseException

That seems to correspond to the following line in uasyncio/__init__.py:

yield IORead(s)

What's going on here?

Start with uasyncio/init.py at the reported line 246, then trace how control.py:main and snakecharmer/main.py handle Ctrl-C. Compare the behavior before and after micropython-lib revision 9e21d6e. Done means explaining why the interrupt reaches this error and identifying the affected change or correction.