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/ValidCheckingInfoObject.py

@@ -17,7 +17,7 @@
 import os
 from Common.RangeExpression import RangeExpression
 from Common.Misc import *
-from StringIO import StringIO
+from io import BytesIO
 from struct import pack
 from Common.DataType import *
 
@@ -162,7 +162,7 @@ class VAR_CHECK_PCD_VARIABLE_TAB_CONTAINER(object):
                             Buffer += b
                             realLength += 1
         
-        DbFile = StringIO()
+        DbFile = BytesIO()
         if Phase == 'DXE' and os.path.exists(BinFilePath):
             BinFile = open(BinFilePath, "rb")
             BinBuffer = BinFile.read()