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/Source/Python/AutoGen/StrGather.py

@@ -18,7 +18,7 @@ import re
 import Common.EdkLogger as EdkLogger
 from Common.BuildToolError import *
 from UniClassObject import *
-from StringIO import StringIO
+from io import BytesIO
 from struct import pack, unpack
 from Common.LongFilePathSupport import OpenLongFilePath as open
 
@@ -341,7 +341,7 @@ def CreateCFileContent(BaseName, UniObjectClass, IsCompatibleMode, UniBinBuffer,
         if Language not in UniLanguageListFiltered:
             continue
         
-        StringBuffer = StringIO()
+        StringBuffer = BytesIO()
         StrStringValue = ''
         ArrayLength = 0
         NumberOfUseOtherLangDef = 0