Digitalarchive

What's Changed
* Compatibility updates for py38-py311.

**Full Changelog**: https://github.com/epikulski/digitalarchive/compare/v0.1.11...v0.1.12

* Re-implement `digitalarchive.models` using pydantic models instead of dataclasses.

* Remove python dev branches from automated buil

* Travis now tests examples in documentation

None is now valid input for date search parameters.

start_date` and `end_date` can now be passed as parameters to `models.Document` searches.