[core] Fix typo in module shutdown method handling

I have no idea how we did not notice this until now. Maybe it means the shutdown
interface isn't that useful :)
[tests] Ignore ipython.py

People shouldn't have to install ipython to use willie. It's a 100% optional
module
Tests failing because of it is silly as it doesn't (and can't) have any tests in
it.

I also added a comment to conftest.py explaining the reason for its existence.