fix: keep the backward compatibility for type hint

[mingrammer]

Hi @liancheng @gabriel-tessier . The | for type hint does not work for < Python 3.9. I fixed it with the Union function.

[mingrammer]

This is a hotfix issue, so I should merge it immediately. Check later, please.

[mingrammer]

cc @filipeaaoliveira, @tvqphuoc01

[gabriel-tessier]

@mingrammer
Do we need to support version < 3.9?

3.8 support end 1 month ago:
https://endoflife.date/python

The unit tests run for Python 3.9 and greater.
https://github.com/mingrammer/diagrams/actions/runs/11715386813

My opinion is to stick with the lower version of the tests 3.9.

Note for later update the Readme.
"It requires Python 3.7 or higher, check your Python version first."

[tvqphuoc01]

agree with @gabriel-tessier our pipeline is only scan code with Python version higher 3.8 so I think we don't need to support v3.8

[mingrammer]

Agree with all of you. I didn't check the EOL of 3.8.

I'll update the badge and requirements in the readme. Thank you for checking it.

[mingrammer]

I made a PR: #1064