lint: Remove several unused imports.

scripts/lib/setup_venv.py

@@ -1,6 +1,5 @@
 
 import os
-import sys
 import subprocess
 from scripts.lib.zulip_tools import run, ENDC, WARNING
 from scripts.lib.hash_reqs import expand_reqs

tools/setup/emoji/emoji_setup_utils.py

@@ -9,7 +9,6 @@
 
 from collections import defaultdict
 from itertools import permutations, chain
-import ujson
 
 from six.moves import zip
 from typing import Any, Dict, List, Text

tools/tests/test_linter_custom_check.py

@@ -1,6 +1,5 @@
 import os
 
-from itertools import chain
 from mock import patch
 from unittest import TestCase