diff -r 3eaff87a0a89 -r 76a64c1e5439 mercurial/pycompat.py
--- a/mercurial/pycompat.py	Fri Aug 05 13:56:10 2016 +0200
+++ b/mercurial/pycompat.py	Fri Mar 03 13:04:32 2017 +0530
@@ -96,6 +96,9 @@
     setattr = _wrapattrfunc(builtins.setattr)
     xrange = builtins.range
 
+    def open(name, mode='r', buffering=-1):
+        return builtins.open(name, sysstr(mode), buffering)
+
     # getopt.getopt() on Python 3 deals with unicodes internally so we cannot
     # pass bytes there. Passing unicodes will result in unicodes as return
     # values which we need to convert again to bytes.