Decrease cost of ipynb code path when unneeded (#3748)

IPython is a very expensive import, like, at least 300ms. I'd also
venture that it's much more common than tokenize-rt, which is like 30ms.
I work in a repo where I use black, have IPython installed and there
happen to be a couple notebooks (that we don't want formatted). I know I
can force exclude ipynb, but this change doesn't really have a cost.