Maint: Change development status from Planning to Beta

[MarieSacksick]

Reference Issues/PRs

What does this implement/fix? Explain your changes.

Update development status.
This is displayed on PyPI (among other places) and serves as an indicator for potential users to assess the project's maturity and whether they can rely on it being maintained over the long term.

I propose setting it to Beta - in any wase, I'm pretty sure it's past level 1 :)

Any other comments?