diff -r 47fac1692ede -r 9f70512ae2cf mercurial/chgserver.py
--- a/mercurial/chgserver.py	Sun Nov 10 07:30:14 2019 -0800
+++ b/mercurial/chgserver.py	Fri Nov 08 11:19:20 2019 -0800
@@ -345,9 +345,9 @@
 
 _iochannels = [
     # server.ch, ui.fp, mode
-    (b'cin', b'fin', r'rb'),
-    (b'cout', b'fout', r'wb'),
-    (b'cerr', b'ferr', r'wb'),
+    (b'cin', b'fin', 'rb'),
+    (b'cout', b'fout', 'wb'),
+    (b'cerr', b'ferr', 'wb'),
 ]