Pinning versions of requirements

[digitaldogsbody]

Tox has started showing a deprecation warning for part of pydantic:

./home/mike/projects/iiif-prezi3/iiif_prezi3/base.py:35: DeprecationWarning: copy_on_model_validation should be a string: 'deep', 'shallow' or 'none'

However, changing this value to any from the list caused test failures. It turns out tox is installing the requirements each run in a temporary virtual environment, and thus was installing a newer version of Pydantic than my local virtualenv.

This leads to test inconsistencies etc and I think is best resolved by pinning dependency versions in setup.py