Blacklist pytest 3.2.0 because it breaks us (#3835)

See upstream: https://github.com/pytest-dev/pytest/issues/2644
diff --git a/setup.py b/setup.py
index a7e63bd..a003bc8 100644
--- a/setup.py
+++ b/setup.py
@@ -55,7 +55,7 @@
     setup_requirements.append("cffi>=1.7")
 
 test_requirements = [
-    "pytest>=2.9.0",
+    "pytest>=2.9.0,!=3.2.0",
     "pretend",
     "iso8601",
     "pytz",