The GitHub labels

Hi team,

We've been discussing at several occasions of cleaning up our GitHub
labels, and merging some. In order to guide the discussion of which
labels to keep and to move, I've analysed a bit what where the labels
we were using. Attached is a plot of the number of issues (including
PRs) open and closed for each labels.
Maybe we have too many labels? Most of them don't seem actively used.
For example, py3k seem to be used 4 times, only on closed tickets/PR.

I've done a similar analysis on scikit-image, scikit-learn and
ipython. Some labels seem to be consistently used quite highly across
all projects: the 3 difficulty levels, bug, ENH, DOC.

I'm playing a bit with the github API this afternoon, so if any other
statistics is useful, just let me know.