pytest migration: models: switch to standard assert statements

Use unittest2pytest to replace unittest-style assert statements (e.g.
assertEqual) with standard Python assert statements to benefit from pytest's
improved reporting on assert failures.

The conversion by unittest2pytest was correct except for:
- 'import pytest' is not automatically added when needed
- line wrapping in string formatting caused a syntax error in the
transformed code. Reported upstream at
https://github.com/pytest-dev/unittest2pytest/issues/3 .
- in assertRaises with a lambda, the lambda needs to be removed