Replace Pydantic ValueError with standard ValueError

[spwoodcock]

The current implementation of pydantic_core.ValidationError did not work.
If the except was triggered here

        try:
            user_data = deserializer.loads(decoded_token)
        except (SignatureExpired, BadSignature) as e:

we simply got: no constructor defined.

This is because pydantic ValidationError cannot be used directly.

Correct usage would be via the from_exception_data static method:

raise ValidationError.from_exception_data(
                {
                    "type": "value_error",
                    "loc": ("access_token",),
                    "input": access_token,
                    "ctx": {
                        "error": "Auth token is invalid or expired",
                    },
                }
            )

However, IMO, this is overly complicated and we don't need to use pydantic for it.
A simple ValueError will suffice & is more maintainable.

Additional context

• We only use Pydantic for two very simple models.
• Pydantic could probably be refactored out eventually, reducing the dependencies required to install this package.