Python 3 compatibility

[josh-t-epic]

This PR adds python 3 compatibility to the Shotgun Python API. Specifically, these changes remove support for python 2.6 and add support for python 3.6+.

Things to consider during review:

• A requirement on six has been added. SG team may want to bundle six with the API rather than adding a requirement.
• Removed legacy support for python 2.4 - 2.6. This means removing some of the bundled modules that are now available in python 2.7+.
• Added an updated httplib2 with python 3 support.
• Nose tests run successfully with python 2.7, 3.6, and 3.7
• Some of the classes that were included in the 4k+ lines shotgun.py have been broken out into their own files for maintainability (and sanity).
• The errors exported from the top-level shotgun_api3 are now exported via shotgun_api3.errors. This wasn't originally part of the plan, but some of the xmlrpclib errors exported here are not available via python 3 and this felt like a good opportunity to clean this up. It is also a bit strange that the Exceptions were being exported via the package like this. This is a backward incompatible change, so will require scrutiny from SG team.
• In order to more easily sift through shotgun.py with PyCharm's inspections to find python3 compatibility (and actual code issues), a pass was done to make shotgun.py PEP-8 compliant.

[jfboismenu]

We've released our official implementation a while back which preserves backwards compatibility with the previous versions of the API.