Update cli_helper.py

[julienschuermans]

Leverage Type Aliases from the typing module.

This avoids throwing an error in python 3.8 when calling pystructurizr dev --view <filename>:

Traceback (most recent call last):
  File "/path/to/venv/bin/pystructurizr", line 5, in <module>
    from pystructurizr.cli import cli
  File "/path/to/venv/lib/python3.8/site-packages/pystructurizr/cli.py", line 9, in <module>
    from .cli_helper import (ensure_tmp_folder_exists, generate_diagram_code_in_child_process, generate_svg)
  File "/path/to/venv/lib/python3.8/site-packages/pystructurizr/cli_helper.py", line 11, in <module>
    def generate_diagram_code_in_child_process(view: str) -> tuple[dict, list[str]]:
TypeError: 'type' object is not subscriptable