add StaticDotMap

[FelixSchwarz]

This is a small utility class I wrote a long time ago. I disliked setting a "privat" attribute every time.

The class behaves exactly like DotMap(_dynamic=False) (which is no wonder if you look at the actual implementation). However this means:

• adding new attributes via __setitem__ still works
• hence the class must be typed as MutableMapping

I'm open to changing it but that might be a bit of work.

Disclaimer: Tests written by Claude with a bit of manual tweaking afterwards.

[FelixSchwarz]

@drgrib Any comments on that addition? If you don't have an opinion, I would like to merge this.