Remove py<1.5 restriction

Fix #2926

changelog/2926.bugfix

@@ -0,0 +1 @@
+Remove ``py<1.5`` restriction from ``pytest`` as this can cause version conflicts in some installations.

setup.py

@@ -43,7 +43,7 @@ def has_environment_marker_support():
 
 
 def main():
-    install_requires = ['py>=1.4.33,<1.5', 'setuptools']  # pluggy is vendored in _pytest.vendored_packages
+    install_requires = ['py>=1.4.33', 'setuptools']  # pluggy is vendored in _pytest.vendored_packages
     extras_require = {}
     if has_environment_marker_support():
         extras_require[':python_version=="2.6"'] = ['argparse', 'ordereddict']