Migrate to pydantic rather than attrs

[holtgrewe]

Is your feature request related to a problem? Please describe.
While attrs is a nice package for defining data classes, pydantic can perform validation and arguably plays better with the Python type (annotation) system.

Describe the solution you'd like
Migrate the code to using pydantic rather than attrs.

Describe alternatives you've considered
N/A

Additional context
N/A