Remove pydantic < v2.0 pin

Removes the pin on pydantic<2.0. We keep using the V1 functionality for now by using from pydantic.v1 import ...

Note: to resolve a "guarded type import" warning in the docs build, this MR also switches to autoapiclass for showing pydantic models in the documentation, which does not work well with the :inherited-members: option, meaning we now have to explicitly list all :members: we want to show in these directives (see #439)

Closes #437 (closed)