fix README open with encoding issue in python 2

Summary: following pull request #834, opening the README file in python 2 fails due to a different API of the `open` function. Using `io.open` fixes the issue.

Reviewed By: EdouardGrave

Differential Revision: D16019456

fbshipit-source-id: 107ff1017508b282524f804bd086e712715a6be3

Author: Celebio

setup.py

@@ -19,6 +19,7 @@
 import os
 import subprocess
 import platform
+import io
 
 __version__ = '0.9'
 FASTTEXT_SRC = "src"
@@ -160,7 +161,7 @@ def _get_readme():
     Use pandoc to generate rst from md.
     pandoc --from=markdown --to=rst --output=python/README.rst python/README.md
     """
-    with open("python/README.rst", encoding='utf-8') as fid:
+    with io.open("python/README.rst", encoding='utf-8') as fid:
         return fid.read()