remove python 2 compatibility

[surrealcristian]

This PR attempt to implement the last bullet in the list of tasks needed to release gunicorn 20.

If something needs to be fixed or changed please let me know.

[jonashaag]

This breaks the build for Python

• 2.6, 2.7 (remove from Travis)
• 3.4 (syx typo)
• 3.5 (issue with current Travis config, fixed by Fix Python 3.5 on Travis typo #1395)

[berkerpeksag]

Thanks for your work, but this more or less a duplicate of #1288.