Drop py2.6 support

[goodboy]

As the drop from pytest via pytest-dev/pytest#2812 we can now remove lingering code that is only in place to support old python.

See pytest-dev/pytest#2822 for comparable CI related changes that are likely appropriate.

Some stuff I can think of off hand includes:

• set logic for checking missing hookcall args can be modernized to use new {,} declarations and dict.viewkeys().
• str.format() method now supports automatic numbering in py2.7+
• leveraging collections.OrderedDict to simplify some of the internal data structures for managing hooks