MAINT: Clean up downsampling boilerplate.

Consolidate docs and mixin applications into one place.

tests/utils/test_preprocess.py

@@ -262,7 +262,11 @@ class PreprocessTestCase(TestCase):
         expected_message = (
             "{qualname}() expected a value in {set_!r}"
             " for argument 'a', but got 'c' instead."
-        ).format(set_=set_, qualname=qualname(f))
+        ).format(
+            # We special-case set to show a tuple instead of the set repr.
+            set_=tuple(set_),
+            qualname=qualname(f),
+        )
         self.assertEqual(e.exception.args[0], expected_message)
 
     def test_expect_dtypes(self):