[lib] Add cached-property requirement for python 2.7

library/setup.py

@@ -133,7 +133,8 @@ setup(
     long_description=open('README.md').read(),
     install_requires=[
         "networkx<2", "lxml",
-        "pycryptosat>0.1.2", "antlr3-python-runtime==3.4"
+        "pycryptosat>0.1.2", "antlr3-python-runtime==3.4",
+        "cached-property==1.5",
     ],
 
     # Tests