Replace self with cls in classmethods for consistency (#211)

Author: thangdangminh

openai/api_resources/customer.py

@@ -3,7 +3,7 @@
 
 class Customer(OpenAIObject):
     @classmethod
-    def get_url(self, customer, endpoint):
+    def get_url(cls, customer, endpoint):
         return f"/customer/{customer}/{endpoint}"
 
     @classmethod

openai/api_resources/moderation.py

@@ -7,7 +7,7 @@ class Moderation(OpenAIObject):
     VALID_MODEL_NAMES: List[str] = ["text-moderation-stable", "text-moderation-latest"]
 
     @classmethod
-    def get_url(self):
+    def get_url(cls):
         return "/moderations"
 
     @classmethod