Use PEP 604 syntax wherever possible (#7493)

stdlib/unittest/suite.pyi

@@ -1,8 +1,8 @@
 import unittest.case
 import unittest.result
-from typing import Iterable, Iterator, Union
+from typing import Iterable, Iterator
 
-_TestType = Union[unittest.case.TestCase, TestSuite]
+_TestType = unittest.case.TestCase | TestSuite
 
 class BaseTestSuite(Iterable[_TestType]):
     _tests: list[unittest.case.TestCase]