reducing nbclean dependencies

[betatim]

In a circleCI with their python:3.7 image I ended up with ipython v7 and an old version of prompt_toolkit. I haven't yet tracked down what is causing the conflict, limiting myself to ipython 6 seems to do the trick for now.

This made me look at the dependencies that were being installed. Which made me wonder: does nbclean need to depend on numpy and tqdm and nbgrader?

TL;DR: can we work on reducing the dependencies of nbclean to a very small number of libraries that are very actively maintained?

[choldgraf]

I'm all for it - the heaviest dependency is definitely nbgrader, which has the logic for replacing text etc (and is the reason nbclean uses traitlets syntax which I'm not a huge fan of TBH). I do agree this could probably be reduced and would make the package more lightweight/maintainable

[betatim]

Let's see if we can extract/make our own version of the text replacement.

[choldgraf]

PRs are welcome ;-)