--- a/mercurial/extensions.py	Wed Jun 13 16:22:54 2018 +0530
+++ b/mercurial/extensions.py	Sun Jun 17 17:59:12 2018 +0900
@@ -605,7 +605,7 @@
 def _disabledhelp(path):
     '''retrieve help synopsis of a disabled extension (without importing)'''
     try:
-        file = open(path)
+        file = open(path, 'rb')
     except IOError:
         return
     else: