Move to `orjson`

[ghinch]

UltraJSON has been put in to "maintenance-only" status and the maintainers recommend upgrading to orjson. orjson is quite a bit faster than ujson, but does have some API incompatibilities.

I'm working on a project right now where we are caching lots of SpaCy objects, and the numerous calls to json_dumps can slow things down. orjson looks to be about twice as fast as ujson on average, and would make a big difference in our case!