Making sure the assert does not fail when required=False, read_only=True

rest_framework/fields.py

@@ -133,7 +133,7 @@ class WritableField(Field):
         if required is None:
             self.required = not(read_only)
         else:
-            assert not read_only, "Cannot set required=True and read_only=True"
+            assert not (read_only and required), "Cannot set required=True and read_only=True"
             self.required = required
 
         messages = {}