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/CapsuleData.py

@@ -17,7 +17,7 @@
 #
 import Ffs
 from GenFdsGlobalVariable import GenFdsGlobalVariable
-import StringIO
+from io import BytesIO
 from struct import pack
 import os
 from Common.Misc import SaveFileOnChange
@@ -82,7 +82,7 @@ class CapsuleFv (CapsuleData):
         if self.FvName.find('.fv') == -1:
             if self.FvName.upper() in GenFdsGlobalVariable.FdfParser.Profile.FvDict:
                 FvObj = GenFdsGlobalVariable.FdfParser.Profile.FvDict[self.FvName.upper()]
-                FdBuffer = StringIO.StringIO('')
+                FdBuffer = BytesIO('')
                 FvObj.CapsuleName = self.CapsuleName
                 FvFile = FvObj.AddToBuffer(FdBuffer)
                 FvObj.CapsuleName = None