Lock-in pytest deps

setup.py

@@ -78,8 +78,9 @@ tests_require = [
     'cqlsh',
     'datadog',
     'httpretty',
-    'pytest-cov>=1.4',
-    'pytest-timeout',
+    'pytest-cov>=1.8.0,<1.9.0',
+    'pytest-timeout>=1.0.0,<1.1.0',
+    'pytest-xdist>=1.11.0,<1.12.0',
     'python-coveralls',
     'responses',
 ]