The djblets.webapi.responses classes (WebAPIResponse,

WebAPIResponsePaginated, WebAPIResponseError,

WebAPIResponseFormError) are core to our API code. These convert

internal structured API response data to payloads for the client.
Although this code rarely needs to change, much of this code is very

old, in need of some love, and in need of unit testing.
To assist in some in-progress work on the API and code using the API,

this change adds type hints to this code, and changes constructors to

require keyword-only arguments (with deprecated compatibility with

positional arguments).
In the process, the documentation for these classes have been fleshed

out.
This did uncover some flaws in WebAPIResponsePaginated where various

methods would crash if queryset was None, or when passing obj to

the constructor. Both have been fixed.
Some unit tests have been added for construction, but many tests are

still lacking.

All unit tests pass.
No issues reported by mypy or pyright.