Fix deprecation errors on pytest v4.0.x

[florisla]

PyTest version 4.0.0 is released and this is breaking both Travis and AppVeyor on deprecation errors.

I stumbled into this in #41.

The main issue seems to be this one: https://docs.pytest.org/en/latest/deprecations.html#marks-in-pytest-mark-parametrize .

You can turn these errors into warnings through a pytest.ini file (see 29b6f90) , but that's only temporary since 4.1.x will break hard on these deprecations.

So we need to port to pytest 4.x. Another option is to pin the pytest version to <4.0.0.