At the moment these tests are skipped ad-hoc in the test suite, but we
can use `pytest.skip` in the context manager itself to skip them
automatically and make things easier on ourselves.

This was originally added when we were still supporting unittest-only
tests, but now that we require pytest, we can switch over to using the
built-in pytest.warns context manager.

Discovered running flake8.

I would humbly like to suggest dateutil drop support for Python 2.6.

The last release of Python 2.6 was 2013-10-29, over 3 years ago. It is
no longer receiving security fixes.

https://www.python.org/dev/peps/pep-0361/

The pip project itself has recently dropped support for 2.6. Their
numbers estimate that Python 2.6 accounts for ~2% of their downloads.

pypa/pip#4343

For projects that still use Python 2.6, they can continue to pip install
an older version.

I've tried my best to remove as much 2.6 specific code as I can,
including the 'Programming Language :: Python :: 2.6' trove classifier
from setup.py. I've also removed Travis CI testing, which should result
in faster testing and fewer wasted resources.

Code changed:

- Removed Python2.6 from testing configuration
- setup.py cleanups due to fewer version complications
- Removed unittest2 dependency and monkey patching
- Use set literals
- Use dict comprehension
- Remove total_seconds workaround
- Remove TarFile.open() context manager workaround

Thanks for considering.

Brings code closer to flake8 compliance by passing rule:

E305 expected 2 blank lines after class or function definition

http://pycodestyle.pycqa.org/en/latest/intro.html#error-codes