Add 3.6 support in setup.py

[lucywang000]

hi, I'm trying to learn about frontera and I hope this can be my a starting point of small contribution.

Currently in setup.py only python 3.5 is claim to be supported, I think we can add 3.6 too.

There are a few tests errors in travis ci, and I don't intent to fix them here - maybe for another pr - imo as long as there are same number of fails for python 3.5 and 3.6 then we're good.

Errors for py35 (build log https://travis-ci.com/lucywang000/frontera/jobs/180100043)

Errors for py36(build log https://travis-ci.com/lucywang000/frontera/jobs/180100044)

For 3.7 there is a problem: thriftpy can't install under 3.7, and that project is no longer maintained:

 thriftpy/transport/cybase.c:3192:11: error: ‘PyThreadState {aka struct _ts}’ has no member named ‘exc_traceback’
       tstate->exc_traceback = tb;
             ^
  error: command 'gcc' failed with exit status 1
  
  ----------------------------------------
  Failed building wheel for thriftpy

but there is a new fork called thriftpy2 (see Yelp/py_zipkin#115). This may also be the work of a separate pr.

[sibiryakov]

Hi @lucywang000 it looks good, but I think we should fix the test before merging this, there could be issues connected with Python3.6 but we're unable to find them unless tests are fixed.

[suzil]

This PR can be closed as there is now 3.6 + 3.7 support from #379.