Drop support for Python 2.6

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.