Use slots where possible

[clokep]

This is somewhat silly, but should be a small perf / memory win for little effort.

Some of these classes get built frequently so might actually be a "real" improvement, but I wouldn't expect any of this to improve things in a noticeable way.

This also gets rid of __nonzero__, which was a Python 2 thing.

[anoadragon453]

LGTM. I read through the attrs docs on slots and the only thing I was worried about was that slots=True means that we can no longer simply assign variables to objects externally.

However, I think we don't do this often if at all, and our tests still seem to pass, so it's probably ok.

[clokep]

LGTM. I read through the attrs docs on slots and the only thing I was worried about was that slots=True means that we can no longer simply assign variables to objects externally.

Right, it means the shape of the object cannot be dynamically modified.

However, I think we don't do this often if at all, and our tests still seem to pass, so it's probably ok.

I did look into how we use each of these and didn't see any spots where we dynamically modify attributes! I should note that this is the reason I did not use these for any of the attrs objects in synapse.config since those are usually constructed by splatting the dictionaries from YAML and it didn't seem like a nice error would occur if an additional parameter was added.

I also skipped a few other objects that are instantiated once or twice (log observers).

[anoadragon453]

@clokep Awesome, I had assumed you checked them already yeah 👍