Qtconsole exit

[takluyver]

Typing 'exit' at the Qt console had stopped working, although 'Exit' still worked. The prefilter system wasn't picking 'exit' up as an automagic, because of the presence of the builtin 'exit' function, which is removed before user code runs.

This does the prefiltering inside our builtin_trap context manager, so it doesn't see the exit builtin, and detects it as an automagic instead.

[fperez]

Thanks a lot! I rebased it on top of Robert's to avoid an unnecessary merge handle in the DAG, but content is unchanged. Commited as 67effcc.