Tkinter moved to separate output

[Ludovic Courtès]

In ‘tk-update’ the ‘python’ packages now have a separate ‘tk’ output for
Tkinter (adding ~50 MiB to the closure of Python.)  I’ve tested it along
these lines, for 2.x and 3.x:

  guix environment --ad-hoc python python:tk --pure -- python
  > import Tkinter
  > root = Tkinter.Tk()
  
One thing I haven’t checked is whether some packages need to have an
additional python:tk input.  Matplotlib maybe?  Federico?

I plan to have Hydra build that branch once we’ve checked that.

Thanks,
Ludo’.