[Py3] fix py3 issue in tools

Bug: v8:9871
Change-Id: I4084771cfc4d34f8e1b9d5265e115f9eac1098d8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2646165Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Gus Caplan <snek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72293}

tools/gen-v8-gn.py

@@ -5,10 +5,15 @@
 # found in the LICENSE file.
 
 import argparse
-from io import BytesIO
 import os
 import sys
 
+if (sys.version_info >= (3, 0)):
+  from io import StringIO
+else:
+  from io import BytesIO as StringIO
+
+
 def parse_args():
   global args
   parser = argparse.ArgumentParser()
@@ -60,7 +65,7 @@ def generate_header(out):
 
 def main():
   parse_args()
-  header_stream = BytesIO("")
+  header_stream = StringIO("")
   generate_header(header_stream)
   contents = header_stream.getvalue()
   if args.output: