pylightning: Fixing incompatibility issue with Python 2.7

Global open doesn't have argument `encoding` in Python 2.7.
open from the io package is needed.

contrib/pylightning/setup.py

@@ -1,8 +1,9 @@
 from setuptools import setup
 import lightning
+import io
 
 
-with open('README.md', encoding='utf-8') as f:
+with io.open('README.md', encoding='utf-8') as f:
     long_description = f.read()
 
 setup(name='pylightning',