Use PEP 604 syntax wherever possible (#7493)

stdlib/dbm/dumb.pyi

@@ -1,11 +1,11 @@
 from _typeshed import Self
 from types import TracebackType
-from typing import Iterator, MutableMapping, Union
+from typing import Iterator, MutableMapping
 
 __all__ = ["error", "open"]
 
-_KeyType = Union[str, bytes]
-_ValueType = Union[str, bytes]
+_KeyType = str | bytes
+_ValueType = str | bytes
 
 error = OSError