Add python3.7 support and remove forgetten 3.3 parts

[usta]

In general just fixes test configs and setup.py , trivial changes

[timabbott]

@usta thanks for the contribution. Can you figure our the CI failure?

[usta]

@timabbott It was a problem about travisCI without using xenial and sudo rights python 3.7 CI couldn't work. ( Python 3.7 requires OpenSSL 1.0.2+ and only xenial have that version )
So until xenial became default we have 2 options :
1- whether we need to especially run 3.7 tests on xenial others on trusty (just like right now )
2- OR I will change it to run all tests ( 2.7 , 3.4 , 3.5 , 3,6 , 3.7 ) will run on xenial

[timabbott]

I think it'd be reasonable to just upgrade to Xenial for CI, since Trusty will be EOL soon.

[usta]

@timabbott so this PR done these things :

• Move CI system to Xenial that will enable us to test 3.7 and others together and much recent platform
• Adds py 3.7 support for setup.py
• Adds py37 env into tox.ini
• Removes 3.3 and adds 3.6 and 3.7 versions into test/init.py

[timabbott]

Merged as the commits ending with e49954e, thanks @usta!

[timabbott]

@eeshangarg can you do a release to offer 3.7 support soon?