diff -r 00c9ac4ce816 -r 277f4fe6d01a mercurial/pycompat.py
--- a/mercurial/pycompat.py	Fri Oct 21 00:09:38 2016 +0900
+++ b/mercurial/pycompat.py	Thu Oct 20 23:40:24 2016 +0900
@@ -44,6 +44,12 @@
     ospathsep = os.pathsep.encode('ascii')
     ossep = os.sep.encode('ascii')
 
+    # TODO: .buffer might not exist if std streams were replaced; we'll need
+    # a silly wrapper to make a bytes stream backed by a unicode one.
+    stdin = sys.stdin.buffer
+    stdout = sys.stdout.buffer
+    stderr = sys.stderr.buffer
+
     # Since Python 3 converts argv to wchar_t type by Py_DecodeLocale() on Unix,
     # we can use os.fsencode() to get back bytes argv.
     #
@@ -100,6 +106,9 @@
     osname = os.name
     ospathsep = os.pathsep
     ossep = os.sep
+    stdin = sys.stdin
+    stdout = sys.stdout
+    stderr = sys.stderr
     sysargv = sys.argv
 
 stringio = io.StringIO