make_docs: Pass the file encoding to open()

Otherwise, it will fail on Python 3 with:
"UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position
7690: ordinal not in range(128)"

We have to use io.open() instead of open(), because open() does not
accept the encoding on Python 2.

Change-Id: I77b6552491e8de01d79a26d2f146ddbd968cce49

scripts/make_docs.py

diff --git a/scripts/make_docs.py b/scripts/make_docs.py
index 42b499a..bb51620 100644
--- a/scripts/make_docs.py
+++ b/scripts/make_docs.py
@@ -35,6 +35,7 @@
 
 from __future__ import print_function
 import glob
+import io
 import os
 import sys
 
@@ -102,7 +103,7 @@
 
 for name in glob.glob('%s/*.html' % html_dir):
   print('Postprocessing: ', name)
-  with open(name) as fptr:
+  with io.open(name, encoding="utf-8") as fptr:
     out = fptr.read()
 
   for input_pattern, output_pattern in replacements:
@@ -119,5 +120,5 @@
   except ValueError:
     print('Skipping breadcrumb strip for', name)
 
-  with open(name, 'w') as fptr:
+  with io.open(name, 'w', encoding="utf-8") as fptr:
     fptr.write(out)