Code completion: Complete on dictionary keys

[dpsanders]

Feature request: add code completion for dictionaries and dictionary-alikes, so that we can do

  d = {"H": 1, "He": 2}
  d[<TAB>

and get a pop-up of available keys.

This would be great, for example, for the matplotlib.settings (formerly [well, OK, still] called matplotlib.rcParams) object, which is a validated dictionary containing all of the settings for matplotlib plots:

  from matplotlib import rcParams as settings
  settings[<TAB>

[minrk]

closing as duplicate of #1533