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/GenFds/FfsInfStatement.py

@@ -18,7 +18,7 @@
 #
 import Rule
 import Common.LongFilePathOs as os
-import StringIO
+from io import BytesIO
 from struct import *
 from GenFdsGlobalVariable import GenFdsGlobalVariable
 import Ffs
@@ -1088,7 +1088,7 @@ class FfsInfStatement(FfsInfStatementClassObject):
     def __GenUniVfrOffsetFile(VfrUniOffsetList, UniVfrOffsetFileName):
 
         # Use a instance of StringIO to cache data
-        fStringIO = StringIO.StringIO('')  
+        fStringIO = BytesIO('')
         
         for Item in VfrUniOffsetList:
             if (Item[0].find("Strings") != -1):