py3: use r'' to prevent conversion to bytes by transformer

py3: define __bytes__ for basefilectx class The implementation is shamely copied from the __str__ function

py3: check for bytes instead of str in isinstance

py3: convert kwargs' keys' to str using pycompat.strkwargs() On Python 3, we must have keys of keyword arguments as str.

py3: convert kwargs keys' back to bytes using pycompat.byteskwargs()

py3: use "%d" % val for int rather than pycompat.bytestr Earlier I used pycompat.bytestr() to convert integers to bytes, but we can do b"%d" % val to convert that int to bytes. b'' is already added by the transformer. Thanks to Yuya for suggesting this.