Drop support for Python 3.8 by the end of October

[soininen]

Python 3.8 will meet its end-of-life in October 2024. We should drop the support from Toolbox and related repositories around the same time after which we can start using features available in Python 3.9 in the code. Technically, dropping support for 3.8 means a small change in pyproject.toml but we should notify our users that are still on 3.8 in advance so they have time to react. Perhaps a message in the Event log in case Toolbox notices it is running on 3.8 explaining the situation suffices?

Now:

• Add notification for users still on 3.8 about the incoming change

In late October:

• Update pyproject.toml and relevant documentation
• Start enjoying the benefits 3.9 gives us

[jkiviluo]

In developer call it was agreed that we will also update to v0.9 since this is kind of breaking change.