Enhancement: Drop Python 3.9 support

[Kumzy]

Summary

As 3.9 reaching EOL in October 2025, maybe drop the 3.9 support for 3.0 release.

• Remove Union from typing
• Update pyproject.toml

[provinzkraut]

Let's drop support for 3.0, as we won't releasing it before 3.9's eol, and we can remove a lot of stuff. Can you add this to the roadmap?