fix %gui asyncio with tornado 5

[minrk]

Fixes test failures when tornado runs on asyncio (tornado 5 + Python ≥3.5).

[minrk]

The main issue appears to be related to the fact that asyncio is already running with tornado 5, so the eventloop integration with %gui asyncio is a no-op. There's something we aren't doing right in that case.

[minrk]

I think I got it. We were registering the default interrupt handler when we enter a custom eventloop, but not when we exit. The result was that our interrupt handler would be unregistered, so interrupting an idle kernel after exiting an eventloop. Running the same pre/post hooks around eventlloop that we use on regular handlers should ensure the right signal handlers are registered appears to fix the issue.

This has been a longstanding bug, but as a bit of a coincidence we just started testing this particular circumstance in #281.