Win32 input for py3: streams are unicode not bytes

On Python 2, streams are bytes, so the unicode data returned from the
win32 api needs to be encoded (as utf-8, in this case). In Python 3,
this is all unicode, so we need to skip the encoding step.

Decoding works OK due to the 'if isinstance(x, unicode)' blocks.

Bug: T76236
Change-Id: I4121ac87816b144e334e7e47c61ce445f0b6e2c9