tests: alphabetize and group Python imports & add check

- De-duplicates, groups, and sorts imports based on the provided `isort` settings.

tests/http/testenv/client.py

@@ -29,12 +29,11 @@ import os
 import shutil
 import subprocess
 from datetime import datetime
-from typing import Optional, Dict
+from typing import Dict, Optional
 
 from . import ExecResult
 from .env import Env
 
-
 log = logging.getLogger(__name__)