Issue #14664: It is now possible to use @unittest.skip{If,Unless} on a test class that doesn't inherit from TestCase (i.e. a mixin).

Lib/unittest/case.py

@@ -61,7 +61,7 @@ def skip(reason):
     Unconditionally skip a test.
     """
     def decorator(test_item):
-        if not (isinstance(test_item, type) and issubclass(test_item, TestCase)):
+        if not isinstance(test_item, type):
             @functools.wraps(test_item)
             def skip_wrapper(*args, **kwargs):
                 raise SkipTest(reason)