BaseTools ConvertMasmToNasm: Support Python 3
    
    The script is updated to support both python 2.7 and python 3.
    
    v2:
     * Use io.open() rather than open() (Jaben)
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Yonghong Zhu <yonghong.zhu@intel.com>
    Cc: Liming Gao <liming.gao@intel.com>
    Cc: Erik Bjorge <erik.c.bjorge@intel.com>
    Reviewed-by: Liming Gao <liming.gao@intel.com>
    Reviewed-by: Erik Bjorge <erik.c.bjorge@intel.com>