Use TypeAlias where possible for type aliases (#7630)

stdlib/unittest/suite.pyi

@@ -1,8 +1,9 @@
 import unittest.case
 import unittest.result
 from typing import Iterable, Iterator
+from typing_extensions import TypeAlias
 
-_TestType = unittest.case.TestCase | TestSuite
+_TestType: TypeAlias = unittest.case.TestCase | TestSuite
 
 class BaseTestSuite(Iterable[_TestType]):
     _tests: list[unittest.case.TestCase]