Mark the minimum pytest version in setup.py. (#3035)

Fixes #3034
diff --git a/setup.py b/setup.py
index 4b63250..3bacae7 100644
--- a/setup.py
+++ b/setup.py
@@ -58,7 +58,7 @@
     setup_requirements.append("cffi>=1.4.1")
 
 test_requirements = [
-    "pytest",
+    "pytest>=2.9.0",
     "pretend",
     "iso8601",
     "pyasn1_modules",