Adding python 3.6 to CI builds

[bsipocz]

Probably we need to wait a bit more until all the dependencies become available on python 3.6.

Edit: this is now based on #869

[bsipocz]

It seems that we have genuine incompatibilities with py3.6:
https://travis-ci.org/astropy/astroquery/jobs/195195678

[keflavich]

Apparently py3.6 doesn't support \s as a generic whitespace character any more? That doesn't show up in the changelog... https://docs.python.org/3.6/whatsnew/3.6.html#re

[keflavich]

Or... \s was always ignored, and now py3.6 is actually telling us: https://docs.python.org/3/library/re.html#regular-expression-syntax "Changed in version 3.6: Unknown escapes consisting of '\' and an ASCII letter now are errors."

[bsipocz]

Maybe it's enough to add an r in front of the strings, as it was done for astropy https://github.com/astropy/astropy/pull/5489/files

[bsipocz]

OK, so there is one more error left. It seems to be an upstream pytest issue, and while we use the astropy bundled version there isn't a chance to avoid it. (Also it's not yet fixed in pytest itself as of pytest-dev/pytest#2160, so it would probably best to find a fix here for now).

I'm not 100% sure, but it seems that using unittest triggers the error, as it's only the esasky module that uses it.

==================================== ERRORS ====================================

_ ERROR collecting lib.linux-x86_64-3.6/astroquery/esasky/tests/test_esasky.py _

_pytest.runner:149: in __init__

    ???

_pytest.main:431: in _memocollect

    return self._memoizedcall('_collected', lambda: list(self.collect()))

_pytest.main:311: in _memoizedcall

    res = function()

_pytest.main:431: in <lambda>

    return self._memoizedcall('_collected', lambda: list(self.collect()))

_pytest.python:600: in collect

    ???

_pytest.python:454: in collect

    ???

_pytest.python:466: in makeitem

    ???

_pytest.vendored_packages.pluggy:724: in __call__

    return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)

_pytest.vendored_packages.pluggy:338: in _hookexec

    return self._inner_hookexec(hook, methods, kwargs)

_pytest.vendored_packages.pluggy:333: in <lambda>

    _MultiCall(methods, kwargs, hook.spec_opts).execute()

_pytest.vendored_packages.pluggy:595: in execute

    return _wrapped_call(hook_impl.function(*args), self.execute)

_pytest.vendored_packages.pluggy:249: in _wrapped_call

    wrap_controller.send(call_outcome)

E   DeprecationWarning: generator 'pytest_pycollect_makeitem' raised StopIteration

[bsipocz]

This now includes #864 to fix most of the deprecations.

[bsipocz]

@keflavich - This can't be merged until the unittests are sorted out. However, since the unittest issue affects only the tests, we can make 0.3.5 py3.6 compatible if I factor out the docstring and other py3.6 related changes from here. Should we do that?

[keflavich]

Sure, if you have the time

[bsipocz]

Merging as finally this passed (except the coverage where something is really badly broken somewhere unrelated to this PR).