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/GenDepex.py

@@ -17,7 +17,7 @@ import Common.LongFilePathOs as os
 import re
 import traceback
 from Common.LongFilePathSupport import OpenLongFilePath as open
-from StringIO import StringIO
+from io import BytesIO
 from struct import pack
 from Common.BuildToolError import *
 from Common.Misc import SaveFileOnChange
@@ -345,7 +345,7 @@ class DependencyExpression:
     #   @retval False   If file exists and is not changed.
     #
     def Generate(self, File=None):
-        Buffer = StringIO()
+        Buffer = BytesIO()
         if len(self.PostfixNotation) == 0:
             return False