Remove some python2 compatibility code, imports and dependencies

[duboism]

As python2 is no longer used, this PR removes some dependencies, imports and code needed to run on both python2 and python3.

Changes in the code are minor:

• import compatibility
• dropping six implies minor modifications
• removal of stat.filemode backported from python 3.4

[duboism]

Set the PR ready for review.

[osallou]

Thanks for your work

[duboism]

You're welcome (and thanks for merging). I have found similar issues in other parts of biomaj so similar PR will come :)