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/OptionRom.py

@@ -17,7 +17,6 @@
 #
 import Common.LongFilePathOs as os
 import subprocess
-import StringIO
 
 import OptRomInfStatement
 from GenFdsGlobalVariable import GenFdsGlobalVariable
@@ -138,5 +137,3 @@ class OverrideAttribs:
         self.PciDeviceId = None
         self.PciRevision = None
         self.NeedCompress = None
-        
-