pep584ify: use dict merge operator (#10279)

This was introduced in Python 3.9. The change makes
dict merges 'obvious' and looks cleaner.

I haven't changed `.update()` to `|=` (update operator),
as I find the former to be more readable and "obvious".

tests/utils/asserts.py

@@ -6,7 +6,7 @@ if TYPE_CHECKING:
 
 
 def issubset(subset: dict, superset: dict) -> bool:
-    assert superset == {**superset, **subset}
+    assert superset == superset | subset
     return True