Pin pytest to 3.3.2

pytest 3.4.0 does not work with tox 2.7 and not with the current version (3.0.0rc1) of tox either

setup.py

@@ -23,7 +23,7 @@
     'pytest-html',
     'pytest-cov',
     'pytest-helpers-namespace',
-    'pytest >= 3.0',
+    'pytest==3.3.2',
     'gitpython',
 ]