python 3.5 compatibility

[csfreebird]

I am using cldoc with python 3.5.2, ran into some errors and fixed them one by one.

1. cldoc/nodes/enum.py的 line 30 and 32
   tokens.next() => next(tokens)
2. cldoc/nodes/function.py
   comment out line 102:
   from namespace import Namespace
3. in cldoc/nodes/field.py, add cmp function

   def cmp(a, b):
    return (a > b) - (a < b) 

[9prady9]

With Python 3.7: Using tokens.__next__() seems to work and I only changed that, didn't have to do additional steps @csfreebird had to do.

Update: nevermind, I got the message much later, I guess we do need the 2 and 3 also. 👍 @csfreebird