RFC: rewrite find_lowest_subclasses to better match its intent

This new implementation is more straightforward with fewer edge cases
that need special handling.

yt/utilities/hierarchy_inspection.py

@@ -1,6 +1,4 @@
 import inspect
-from collections import Counter
-from functools import reduce
 from typing import List, Type
 
 
@@ -23,20 +21,9 @@ def find_lowest_subclasses(candidates: List[Type]) -> List[Type]:
         candidates.
     """
 
-    # If there is only one input, the input candidate is always the
-    # lowest class
-    if len(candidates) == 1:
-        return candidates
-    elif len(candidates) == 0:
-        return []
+    # The MRO list includes the class itself, which we don't want
+    superclasses = [set(inspect.getmro(c)) - {c} for c in candidates]
 
-    mros = [inspect.getmro(c) for c in candidates]
+    all_superclasses = set().union(*superclasses)
 
-    counters = [Counter(mro) for mro in mros]
-
-    if len(counters) == 0:
-        return []
-
-    count = reduce(lambda x, y: x + y, counters)
-
-    return [x for x in candidates if count[x] == 1]
+    return [x for x in candidates if x not in all_superclasses]

yt/utilities/tests/test_hierarchy_inspection.py

@@ -27,6 +27,11 @@ class level4(level3):
     pass
 
 
+def test_empty():
+    result = find_lowest_subclasses([])
+    assert len(result) == 0
+
+
 def test_single():
     result = find_lowest_subclasses([level2])
     assert len(result) == 1
@@ -60,4 +65,10 @@ def test_diverging_tree():
 def test_without_parents():
     result = find_lowest_subclasses([level1, level3])
     assert len(result) == 1
-    assert level3 in result
+    assert result[0] is level3
+
+
+def test_without_grandparents():
+    result = find_lowest_subclasses([level1, level4])
+    assert len(result) == 1
+    assert result[0] is level4