fix: let model_validator return self (#205)

* fix: let `model_validator` return `self`

UserWarning: A custom validator is returning a value other than `self`.
Returning anything other than `self` from a top level model validator isn't supported when validating via `__init__`.
See the `model_validator` docs (https://docs.pydantic.dev/latest/concepts/validators/#model-validators) for more details.

* fix mypy

---------

Co-authored-by: Konstantin <konstantin.klein+github@hochfrequenz.de>

src/tmdsclient/client/config.py

@@ -2,7 +2,7 @@
 contains a class with which the TMDS client is instantiated/configured
 """
 
-from typing import Any
+from typing import Any, Self
 
 from pydantic import BaseModel, ConfigDict, HttpUrl, field_validator, model_validator
 from yarl import URL
@@ -89,23 +89,24 @@ class OAuthTmdsConfig(TmdsConfig):
     This is useful when you have ways to get a token but not a client id and secret.
     """
 
-    @model_validator(mode="after")  # type:ignore[misc, no-untyped-def]
-    def check_secret_or_token_is_present(cls, values) -> None:  # pylint:disable=no-self-argument
+    @model_validator(mode="after")
+    def check_secret_or_token_is_present(self) -> Self:  # pylint:disable=no-self-argument
         """
         Ensures that either (id+secret) or a bare token are present
         """
-        token_is_present = values.bearer_token is not None and values.bearer_token.strip()
+        token_is_present = self.bearer_token is not None and self.bearer_token.strip()
         client_id_and_secret_are_present = (
-            values.client_id is not None
-            and values.client_id.strip()
-            and values.client_secret is not None
-            and values.client_secret.strip()
+            self.client_id is not None
+            and self.client_id.strip()
+            and self.client_secret is not None
+            and self.client_secret.strip()
         )
         if not token_is_present and not client_id_and_secret_are_present:
             raise ValueError(
                 # pylint:disable=line-too-long
                 "You must provide either client id and secret or a bearer token, but not None of both"
             )
+        return self
 
     @field_validator("bearer_token")
     def validate_bearer_token(cls, value: str) -> str: