Make build-script-helper.py pass flake8 linting

In preparation for swiftlang/github-workflows#14

Author: ahoppen

.flake8

@@ -0,0 +1,4 @@
+[flake8]
+
+# Match the maximum line length in Swift files.
+max-line-length = 120

build-script-helper.py

@@ -15,12 +15,12 @@
 from __future__ import print_function
 
 import argparse
-import sys
-import os, platform
+import json
+import os
 import subprocess
+import sys
 from pathlib import Path
-from typing import List, Union, Optional
-import json
+from typing import List, Optional, Union
 
 # -----------------------------------------------------------------------------
 # General utilities