reset readline after running PYTHONSTARTUP

should avoid conflicts between IPython and Python completion setup

Many folks have completion setup in PYTHONSTARTUP for the plain Python REPL. In IPython 1.x, this wasn't an issue because IPython reset the readline state after every execution. This was removed in 2.0 (#4353), allowing users to customize readline behavior. One effect of this is allowing PYTHONSTARTUP to customize *IPython's* readline behavior, which seems to be something nobody wants.

This PR simply re-applies the old reset readline behavior after running PYTHONSTARTUP.

closes #5514