pasteurize: Py3 to Py2/3

Running pasteurize -w mypy3module.py turns this Python 3 code:

import configparser

class Blah:
    pass
print('Hello', end=None)

into this code which runs on both Py2 and Py3:

from __future__ import print_function
from future import standard_library
standard_library.install_hooks()

import configparser

class Blah(object):
    pass
print('Hello', end=None)

Notice that both futurize and pasteurize create explicit new-style classes that inherit from object on both Python versions, and both refer to stdlib modules (as well as builtins) under their Py3 names.

pasteurize also handles the following Python 3 features:

• keyword-only arguments
• metaclasses (using with_metaclass())
• extended tuple unpacking (PEP 3132)

To handle function annotations (PEP 3107), see Function annotations.