Remove most of the python 2 compat code

[rahulporuri]

Note to reviewer : I have tried making one and only one type of change in each of the commits so reviewing this PR commit-by-commit will be straightforward.

This PR removes most of the machinery in the codebase that supported python 2/3 compatibility.

• remove __future__ import statements
• remove coding/encoding cookies from files
• remove try/except statements around imports that have changed between python 2 and 3
• use str instead of py3compat.unicode_type
• use str instead of py3compat.string_types
• use dict.items() instead of py3compat.iteriterms(dict)
• Remove a python 2 only test
• Remove py3compat.PY3 conditionals
• Use metaclass kwarg instead of py3compat.with_metaclass
• Use builtins instead of py3compat.builtin_mod
• Use func.__closure__ instead of py3compat.get_closure(func)
• Remove py3compat.*_py2 functions which are no-op on python 3

[blink1073]

Looks great, thanks! We picked up a few merge conflicts from #538, mind doing a rebase?

[rahulporuri]

@blink1073 i think I rebased correctly and didn't mess anything up.

[blink1073]

Great, thank you!