Python 3.6 compatibility - #717

@bsipocz - I think I'm getting close to python 3.6 compatibility. I've reached a new error that it looks like you solved in an AstroQuery Issue,

DeprecationWarning: generator 'pytest_pycollect_makeitem' raised StopIteration

but in that Issue you say something I don't understand, "The best and easiest way to deal with it in astroquery to change this test to be pure pytest.". Can you elaborate how I should address this?

@aphearin - #648 got rebased, but is still far behind this one and less consistent with the raw strings.

The unittest issue is not yet solved in astroquery, but I'm fairly certain that it is due to some upstream pytest incompatibility with unittest on py3.6 that raises that deprecation warning.
In astroquery we will solve this by getting rid of the few tests that are using unittest by rewriting them to use pytest only (it only affects two recently added module which were mostly copied over from existing ESA software). However for halotools you may want to keep using unittest, so then you need to dig deeper and try to find the real underlying issue within pytest.

A third short term workaround would be to disable deprecation warnings that come from pytest to be raised as exceptions.

@bsipocz - I tried to mirror exactly your previous work on the .travis.yml config file in Halotools in order to set 3.6 as an "Allowed Failure". However, when I look at the Travis run displayed in https://travis-ci.org/astropy/halotools/builds/215933438, it seems that my attempt to run python 3.6 with SETUP_CMD='test' is not executing at all - there is no "Allowed Failure" section that appears in the CI build matrix. Would you mind taking a look to see where I made a mistake in writing the .travis.yml?