pytest 3.6.0 raises a RemovedInPytest4Warning breaking pytest-xdist

... or execnet or something as it can't serialize this exception.

The warning is caused by our usage of a [pytest] section in setup.cfg.
I rather don't want to change this as it might break older pytest
versions.

requirements.d/development.txt

@@ -3,7 +3,7 @@ setuptools_scm
 pip
 virtualenv<14.0
 tox<3.2.0
-pytest!=3.3.0
+pytest<3.6.0
 pytest-xdist
 pytest-cov
 pytest-benchmark