gh-37563: trying a change in unique_representation
    
namely break the loop over `mro` once found what we want

I am not sure at all that this keeps the expected properties, but this
is more efficient..

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #37563
Reported by: Frédéric Chapoton
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/sage/structure/unique_representation.py

@@ -1008,7 +1008,7 @@ def __classcall__(cls, *args, **options):
             True
         """
         instance = typecall(cls, *args, **options)
-        assert isinstance( instance, cls )
+        assert isinstance(instance, cls)
         if instance.__class__.__reduce__ == CachedRepresentation.__reduce__:
             instance._reduction = (cls, args, options)
         return instance
@@ -1090,16 +1090,14 @@ def _clear_cache_(cls):
             sage: c is C(3)
             False
         """
-        del_list = []
         cache = None
+        # not clear why the loop below is taking the last C
         for C in cls.mro():
             try:
                 cache = C.__classcall__.cache
             except AttributeError:
                 pass
-        for k in cache:
-            if issubclass(k[0][0],cls):
-                del_list.append(k)
+        del_list = [k for k in cache if issubclass(k[0][0], cls)]
         for k in del_list:
             del cache[k]
 
@@ -1149,6 +1147,7 @@ def __deepcopy__(self, memo):
         """
         return self
 
+
 def unreduce(cls, args, keywords):
     """
     Calls a class on the given arguments::
@@ -1166,7 +1165,7 @@ def unreduce(cls, args, keywords):
 
 class UniqueRepresentation(CachedRepresentation, WithEqualityById):
     r"""
-    Classes derived from UniqueRepresentation inherit a unique
+    Classes derived from ``UniqueRepresentation`` inherit a unique
     representation behavior for their instances.
 
     .. SEEALSO::