--- a/mercurial/formatter.py	Sat May 06 14:40:22 2017 +0900
+++ b/mercurial/formatter.py	Sat May 06 14:47:31 2017 +0900
@@ -399,7 +399,7 @@
         # is it a mapfile for a style?
         if os.path.basename(tmpl).startswith("map-"):
             return None, os.path.realpath(tmpl)
-        with open(tmpl) as f:
+        with open(tmpl, 'rb') as f:
             tmpl = f.read()
         return tmpl, None