Make numpydoc Python 3 compatible

Issue #2918: Numpydoc (in doc/sphinxext) does not support Python 3.

This pull request aims to solve this issue at least partially.
* setup.py uses 2to3.
* For Python 3, docscrape.py uses inspect.getfullargspec instead of deprecated inspect.getargspec.
* Unicode tests in test_docscrape.py are written separately for Python 2 and 3.

However, in Python 3, nosetests gives error:
  File "/../docscrape_sphinx.py", line 3, in <module>
    from .docscrape import NumpyDocString, FunctionDoc, ClassDoc
ValueError: Attempted relative import in non-package

I am not sure how to solve this problem.