Refactor serializers, add YAML serializer

The Django setup for loading serializers is much more complicated than
this project needs, so let's remove the boilerplate and make it simple.
Freeze pytest-xdist

pytest-dxist 1.9 introduces a change on the loop on fail, so it runs
tests on every file change, even Vim swap files... So freezing the
dependency until they fix it or I can find a better solution.