requirements-dev.txt: add missing pylint for liniting tests

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

requirements-dev.txt

@@ -5,6 +5,7 @@ mock==2.0.0
 nose2[coverage_plugin]
 cov-core==1.15.0
 pep8==1.7.0
+pylint
 plone.testing==5.0.0
 splinter==0.11.0
 transifex-client==0.12.2