ci: Travis: Python 3.5.1 via Trusty

[blueyed]

Python 3.5.0 caused flaky failures before
(#5795), hopefully this is
more stable.

This is pulled out of #6435,
which adds code specific for Python < 3.5.2, and therefore it makes
sense to test it.

[nicoddemus]

Seems still needs work:

  File "/home/travis/build/pytest-dev/pytest/.tox/py35-xdist/lib/python3.5/site-packages/hypothesis/internal/compat.py", line 22, in <module>
    from typing import Tuple, Type  # noqa
ImportError: cannot import name 'Type'

[The-Compiler]

@nicoddemus See my comment here: #6435 (comment)

Should be fixed in hypothesis 5.1.3 which was released today.

[nicoddemus]

Cool, thanks for the link. 👍

[blueyed]

Still fails with hypothesis 5.1.4: HypothesisWorks/hypothesis#2318

[blueyed]

[gw1] linux -- Python 3.5.1 /home/travis/build/pytest-dev/pytest/.tox/py35-xdist/bin/python
worker 'gw1' crashed while running 'testing/test_config.py::test_config_blocked_default_plugins[monkeypatch]'

gw2 then does not re-try the test (xdist issue?).

I think we should run this only with a subset to cover enough of 3.5.{0,1} specific code.

[blueyed]

Should be good now.

[blueyed]

Would like to get this merged for #6435.