Minimum version of pydantic seems not sufficient

[phackstock]

When trying to import ixmp4 (or pyam) in the below example with pydantic version 2.9, we get the following error:

>>> import pyam
Traceback (most recent call last):
  File "/home/hackstock/.local/lib/python3.11/site-packages/ixmp4/db/filters.py", line 336, in __init__
    super().__init__(**data)
  File "/home/hackstock/.local/lib/python3.11/site-packages/pydantic/main.py", line 212, in __init__
    validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 1 validation error for OptimizationScalarFilter
unit__id
  Extra inputs are not permitted [type=extra_forbidden, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.9/v/extra_forbidden

Updating to pydantic v2.12 seems to resolve the issue.