BaseTools ConvertMasmToNasm: Support Python 3

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>