Support iteration over enums with enum34 - #1412

This PR's goal is to support the following in python 2:

from enum import Enum

class Colors(Enum):
   RED = 0
   BLUE = 1

for color in Colors:
  print(color.name)

Iteration over enums in python3 was supported in different PR. (this PR is pretty much a copy of the python 3 one)