PY3 port scrapy.utils.misc.arg_to_iter; fix scrapy.utils.misc.md5sum doctest
TST enable doctests for Python 3
PY3 port scrapy.contrib.feedexport
TST modernize some of the asserts
PY3 port scrapy.link
Merge pull request #835 from scrapy/modernize-asserts

[MRG] modernize some of the asserts
PY3 top-level shortcuts work
Modernize setup.py