[Py3] Fix flake8 warnings

- was missing an import sys
- check for long
- check for xrange

This file is now flake8 warning free, and should work on both Py2 and
Py3.

$ flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics

Bug: v8:8594
Change-Id: Iae857f4686bcad509fa700954b7f30f86150739f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2288177Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68809}

test/debugging/wasm/gdb-server/gdb_rsp.py

@@ -8,6 +8,20 @@ import struct
 import subprocess
 import time
 import xml.etree.ElementTree
+import sys
+
+# Python2 has both int and long, Python3 only has int, which is the same as
+# Python2 long.
+try:
+  long
+except NameError:
+  long = int
+
+# Python2 has xrange and range. Python3 range is Python2 xrange.
+try:
+  xrange
+except NameError:
+  xrange = range
 
 SOCKET_ADDR = ('localhost', 8765)