remove py2 dependence

[00Kai0]

No description provided.

[lresende]

LGTM, but leaving it open for a day or so to give a chance for the original reviewers to say something.
@minrk @takluyver

[00Kai0]

@lresende ,thank you for your approval.
I think I have removed most of py2 dependence.
And I find other problems that should be solved:

• Some code imported but unused.
• Replace gen.coroutine to async/await, because we just want to support >=py3.5.
• Some code don't follow pep 8code style (if we want to follow this style).

[takluyver]

Feel free to open PRs to clean up unused imports, and we can probably convert coroutines to async/await syntax. I'd say try to work piecewise where practical - small to medium PRs are easier to review than large ones.

Follow PEP 8 where you're adding or changing code anyway, but please don't reformat huge amounts of code that you're not otherwise touching - that causes merge conflicts unnecessarily and makes git blame less useful.

[takluyver]

PS: When I say "follow PEP 8", I mean roughly. As you can probably tell, we're not strict about this - if leaving a line a couple of characters over the limit is more readable, for example, that's OK.

[00Kai0]

@takluyver I agree with you.