fix: Assume expiration_timestamp is utc (#135)

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

src/tmdsclient/client/oauth.py

@@ -23,7 +23,7 @@ def token_is_valid(token: str) -> bool:
     try:
         decoded_token = jwt.decode(token, algorithms=["HS256"], options={"verify_signature": False})
         expiration_timestamp = decoded_token.get("exp")
-        expiration_datetime = datetime.fromtimestamp(expiration_timestamp)
+        expiration_datetime = datetime.fromtimestamp(expiration_timestamp).replace(tzinfo=UTC)
         _logger.debug("Token is valid until %s", expiration_datetime.isoformat())
         current_datetime = datetime.now(UTC)
         token_is_valid_one_minute_into_the_future = expiration_datetime > current_datetime + timedelta(minutes=1)