Compatibility fixes for Pytest 4.0

[mixja]

This PR adds compatibility with pytest version 4.0.

Currently pytest-responses does not work with pytest 4.x with the following error raised for each test:

def pytest_runtest_setup(item):
>       if not item.get_marker('withoutresponses'):
E       AttributeError: 'Function' object has no attribute 'get_marker'

The get_marker function has been removed as per pytest-dev/pytest#4546, and this PR replaces this with the get_closest_marker function.

[dcramer]

sentry runs pytest 3.5 right now, our zeus project runs 3.2.x, so requiring a significantly newer version will create headaches for in-house developers

[dcramer]

(we likely can upgrade those, but that will of course block this PR for us)

[blueyed]

Please consider either a) handling older versions also (which should be trivial), or b) create a release with an upper version for the pytest dep.

@mixja
Are you up for adjusting the PR, to still use get_marker if get_closest_marker is not available?

[MRigal]

I guess can be closed as addressed through #9