py3: make encoding.strio() an identity function on Python 2

It's the convention the other encoding.str*() functions follow. To make things
simple, this also drops kwargs from the strio() constructor.

mercurial/encoding.py

@@ -575,15 +575,17 b' def fromutf8b(s):'
         r += c
     return r
 
+if pycompat.ispy3:
 class strio(io.TextIOWrapper):
     """Wrapper around TextIOWrapper that respects hg's encoding assumptions.
 
     Also works around Python closing streams.
     """
 
-    def __init__(self, buffer, **kwargs):
-        kwargs[r'encoding'] = _sysstr(encoding)
-        super(strio, self).__init__(buffer, **kwargs)
+        def __init__(self, buffer):
+            super(strio, self).__init__(buffer, encoding=_sysstr(encoding))
 
     def __del__(self):
         """Override __del__ so it doesn't close the underlying stream."""
+else:
+    strio = pycompat.identity

mercurial/util.py

@@ -175,11 +175,10 b' def safehasattr(thing, attr):'
 def bytesinput(fin, fout, *args, **kwargs):
     sin, sout = sys.stdin, sys.stdout
     try:
+        sys.stdin, sys.stdout = encoding.strio(fin), encoding.strio(fout)
         if pycompat.ispy3:
-            sys.stdin, sys.stdout = encoding.strio(fin), encoding.strio(fout)
             return encoding.strtolocal(input(*args, **kwargs))
         else:
-            sys.stdin, sys.stdout = fin, fout
             return raw_input(*args, **kwargs)
     finally:
         sys.stdin, sys.stdout = sin, sout