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/Fv.py

@@ -17,7 +17,7 @@
 #
 import Common.LongFilePathOs as os
 import subprocess
-import StringIO
+from io import BytesIO
 from struct import *
 
 import Ffs
@@ -265,7 +265,7 @@ class FV (FvClassObject):
         #
         self.InfFileName = os.path.join(GenFdsGlobalVariable.FvDir,
                                    self.UiFvName + '.inf')
-        self.FvInfFile = StringIO.StringIO()
+        self.FvInfFile = BytesIO()
 
         #
         # Add [Options]
@@ -407,7 +407,7 @@ class FV (FvClassObject):
             #
             if TotalSize > 0:
                 FvExtHeaderFileName = os.path.join(GenFdsGlobalVariable.FvDir, self.UiFvName + '.ext')
-                FvExtHeaderFile = StringIO.StringIO()
+                FvExtHeaderFile = BytesIO()
                 FvExtHeaderFile.write(Buffer)
                 Changed = SaveFileOnChange(FvExtHeaderFileName, FvExtHeaderFile.getvalue(), True)
                 FvExtHeaderFile.close()