py3: conditionalize initialization of stdio flags

Since Python 3 doesn't depend on the stdio of libc, there should be no need
to set O_BINARY flag on Windows.

mercurial/dispatch.py

@@ -105,9 +105,12 b' def run():'
         # change the status code and move on.
         except IOError:
             status = -1
-
     sys.exit(status & 255)
 
+if pycompat.ispy3:
+    def _initstdio():
+        pass
+else:
 def _initstdio():
     for fp in (sys.stdin, sys.stdout, sys.stderr):
         util.setbinary(fp)