Define abstract Mutation.mutate() method

Add default implementation of Mutation.mutate() that raises NotImplementedError.
This makes code more clear and also improves work of auto-completion tools (e. g. in IDE). They usually guess if user is overriding a class method and suggest to complete method name/arguments.

graphene/types/mutation.py

@@ -106,9 +106,7 @@ def __init_subclass_with_meta__(
             else:
                 arguments = {}
         if not resolver:
-            mutate = getattr(cls, "mutate", None)
-            assert mutate, "All mutations must define a mutate method in it"
-            resolver = get_unbound_function(mutate)
+            resolver = get_unbound_function(cls.mutate)
         if _meta.fields:
             _meta.fields.update(fields)
         else:
@@ -119,6 +117,10 @@ def __init_subclass_with_meta__(
         _meta.arguments = arguments
 
         super(Mutation, cls).__init_subclass_with_meta__(_meta=_meta, **options)
+
+    @classmethod
+    def mutate(cls, parent, info, **kwargs):
+        raise NotImplementedError("All mutations must define a mutate method in it")
 
     @classmethod
     def Field(