Support python 3

[ryan-williams]

h/t @arahuja

[arahuja]

2to3 seems to do a decent job to update the script, but a dependency seems to be missing:

Traceback (most recent call last):
  File "/Users/arahuja/src/hammerlab/git-helpers/branch/git-branches", line 14, in <module>
    from util.branch_infos import BranchInfos
  File "/Users/arahuja/src/hammerlab/git-helpers/util/branch_infos.py", line 5, in <module>
    from .branch_info import BranchInfo
  File "/Users/arahuja/src/hammerlab/git-helpers/util/branch_info.py", line 5, in <module>
    from color import clen, color
ImportError: No module named 'color'

I can't find where color came from or what it is, any ideas?

[ryan-williams]

color.py actually lives in this repo.

I think I've got this ported, will PR it soon