Fix test for Python 3.8 (#27)

## Summary

TypeError: 'type' object is not subscriptable

## How was this tested?

CI

## PRs in the Stack
- #26
- ➡ #27

(The stack is managed by
[git-grok](https://github.com/dimikot/git-grok).)

Author: dimikot

tests/helpers.py

@@ -1,3 +1,4 @@
+from __future__ import annotations
 import json
 import re
 import shlex