BaseTools: Replace StringIO.StringIO with io.BytesIO

Replace StringIO.StringIO with io.BytesIO to be compatible with python3.
This commit also removes "import StringIO" from those python scripts
that don't really use it.

Contributed-under: TianoCore Contribution Agreement 1.1
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>

BaseTools/Scripts/ConvertUni.py

@@ -23,11 +23,6 @@ import codecs
 import os
 import sys
 
-try:
-    from io import StringIO
-except ImportError:
-    from StringIO import StringIO
-
 class ConvertOneArg:
     """Converts utf-16 to utf-8 for one command line argument.