fix: make MultiPartParser read default max_part_size from the class attr

[reith]

Summary

In FastAPI, the only way to change the maximum part size in a multipart request is to override MultiPartParser.max_part_size globally. That has stopped working since the changes introduced by this commit. I think the change in this PR is benign to keep. Even without considering compatibility with FastAPI, I think falling back to the class attribute to read the default value is a good idea but I don't mind if you think that problem should be addressed separately by FastAPI.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

The class attributes should be removed, they are still there by mistake.

FastAPI should be smart about this.

[Kludex]

That said... I understand it can be a bit annoying to have to wait for FastAPI...

I think we need a constant DEFAULT_MAX_SIZE_... or something like that, which will allow users to change the behavior while waiting for FastAPI.

[reith]

I ended up getting the raw request in my handler and using the Starlette's Request API there to change the part size limit. That’s better than changing the default since I perform authorization in a middleware, and it makes more sense not to raise the limit for unauthenticated requests.

Thanks for being considerate, though!