ChatPromptTemplate is not an ABC, it's instantiated directly. (#9468)

Its own `__add__` method constructs `ChatPromptTemplate` objects
directly, it cannot be abstract.

Found while debugging something else with @nfcampos.

libs/langchain/langchain/prompts/chat.py

@@ -337,7 +337,7 @@ MessageLikeRepresentation = Union[
 ]
 
 
-class ChatPromptTemplate(BaseChatPromptTemplate, ABC):
+class ChatPromptTemplate(BaseChatPromptTemplate):
     """A prompt template for chat models.
 
     Use to create flexible templated prompts for chat models.