[git-cl] Lint and clean-up git-cl, test, and related modules

In this CL:
 - Clarify some comments.
 - Remove some unused imports.
 - Make some style more consistent (e.g. quotes, whitespace)

Tools used: pyflakes, flake8 (most warnings ignored)

Change-Id: Ibfb6733c8d844b3c75a7f50b4f3c1d43afabb0ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1773856
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
diff --git a/gsutil.py b/gsutil.py
index 55e4cb0..64e4b5c 100755
--- a/gsutil.py
+++ b/gsutil.py
@@ -86,6 +86,7 @@
     if os.path.isdir(tmpdir):
       shutil.rmtree(tmpdir)
 
+
 def ensure_gsutil(version, target, clean):
   bin_dir = os.path.join(target, 'gsutil_%s' % version)
   gsutil_bin = os.path.join(bin_dir, 'gsutil', 'gsutil')
@@ -183,5 +184,6 @@
   return run_gsutil(args.force_version, args.fallback, args.target, args.args,
                     clean=args.clean)
 
+
 if __name__ == '__main__':
   sys.exit(main())