Enable strict=True for ValuesAxis

[PaulVanSchayck]

Pydantic v2 strict mode has been enabled everywhere, except for ValuesAxis.

When enabled the following test would fail:

>       assert object_type.model_validate_json(json_string).model_dump_json(exclude_none=True) == json_string
E       pydantic_core._pydantic_core.ValidationError: 4 validation errors for Axes
E       t.values.0
E         Input should be a valid datetime [type=datetime_type, input_value='2008-01-01T04:00:00Z', input_type=str]
E           For further information visit https://errors.pydantic.dev/2.3/v/datetime_type

This seems to be an issue in the combination between Generics (ValuesT) and datetime and strict mode when converting between JSON <-> Pydantic. Needs further investigation.