Remove most py2 compat code

[matthewturk]

This removes most of the python2 compatibility code. There are a
handful of things that we no longer need to do, specifically lots of
class Something(object): invocations.

Unit tests pass; I anticipate there may be issues in answer testing
related to mistakes in importing or decode/encode of strings, and I look
forward to fixing those.

[ngoldbaum]

One thing that I'm a little nervous about on the yt-4.0 branch, especially for a big invasive change like this, is that we don't have great test coverage for the frontends because travis and appveyor don't run tests with real data.

It would be great if we could get Jenkins running against the yt-4.0 branch next week.

[matthewturk]

OK, so this is a non-starter until then?

[ngoldbaum]

Make sure python_requires in setup.py gets updated, it can now just be >=3.5 I think.

[ngoldbaum]

Getting this working would definitely be worthwhile since you'll have to do this annoying mechanical work anyway, there just might be more brokenness in the frontends or in code that only has tests that uses real data that you won't notice until jenkins gets turned on so this probably can't be merged until then.

[matthewturk]

To close the loop, this is dependent on #2208 and #2172 , and then (pending those tests passing) I believe it is ready to go.

[munkm]

I think this is now also dependent on #2260

[brittonsmith]

These all look really straightforward to me. I think this is good.

[jzuhone]

LGTM!

[chummels]

I looked over this, and everything seems to be in order. I think there is just one small thing that could improve this PR. Maybe we could include a check when yt gets imported by including code like this in the __init__.py:

import sys
if sys.version_info[0] < 3:
    raise Exception("Python 2 no longer supported.  Please install Python 3 for use with yt.")

[pep8speaks]

Hi there, @matthewturk! Thanks for updating this PR.

• In the file yt/__init__.py:

Line 21:9: E117 over-indented

[matthewturk]

OK! I have merged, and I think this is ready-for-review again.

I have addressed this, and I think Cameron might be away for a while!