Rely on umask for default directory permissions

[takluyver]

Closes gh-2858

@minrk this feels somewhat unsatisfactory. The function I want to change is in ipython_genutils, but we're not making any changes there. So do we need to extract it to each package that uses it to make similar fixes?

[blink1073]

This search shows that ipython itself is no longer using ipython_genutils outside of indent and dedent in the docs generator. Perhaps moving the functions to jupyter_core makes sense?

[blink1073]

To be clear: I mean copying the functions currently used in this repo to jupyter_core, releasing jupyter_core, and removing the dependency of this repo on ipython_genutils.

[takluyver]

That's a good idea. jupyter_core probably makes reasonable sense as a place for this.

[takluyver]

jupyter/jupyter_core#113 would add it to jupyter_core.

[takluyver]

Closing in favour of #3002.