Update to Databind 4.x

[rbroderi]

Relax databind version requirements.

[NiklasRosenstein]

Hey @rbroderi, the latest version of databind (4.x) is not quite compatible with the Docspec codebase. Now that wouldn't be an issue if Pydoc-Markdown (which is a primary consumer of Docspec) wouldn't also be locked onto the same databind version at the moment.

Preparing Pydoc-Markdown to upgrade to the latest databind version might be a bit more involved, because the dependency propagates to plugins implemented for Pydoc-Markdown and there are also some changes in the behaviour that need to be included in some kind of upgrade guide.

It's on my todo list, but as you can probably tell, I'm struggling to find time to continue work on my OSS projects.

Just wanted to give you a heads up!