When building docs, Sphinx tries to recurse into types as far as it can

before bailing. Our JSON types can recurse infinitely, and this

generates some pretty pathological output with long read and write

times.
Fortunately, Sphinx has a way of turning a type reference to a

hard-coded string, which seems to be intended for cases of pathological

types. We now set this when building our docs, ensuring that we give a

meaningful type and don't attempt to recurse.

Successfully built our docs and verified that the djblets.util.typing

docs weren't showing giant recursive types.