Use the builtin "open" on Python 3.
Merge pull request #907 from saimn/py3k

Drop python 3.2 support and move to python 3.3
tests/test_readers.py: Remove redundant 'expected' check in AdReaderTest

AdReaderTest.test_article_with_asc_extension() has had duplicate
checks on the expected metadata since it was created by 49f481e3 (Add
asciidoc reader support, 2013-10-28).  This commit removes the
duplicate entry.
Ensure that markup is a tuple.

`self.markup` is a list when using the `-m|--markup` cli option, but testing the
extension with `endswith` works only with tuples.